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This article is about cloudy, dull, blotch, poor adhesion, cracks, crinkles, and other things that can go wrong with a finish on a piece of wood; how to prevent them; and how to fix them. Along the way, we will discuss the hazards of such things as heat, high humidity, oil, contaminants, and getting in a hurry. All of these problems can happen to any piece of finished wood from pens to salad bowls and beyond, and with any finish from wax to CA glue and whatever else we can put on a piece of wood.

There are no photographs with this article. Anyone who has had any of these problems will already know what they look like and don’t need a picture.

High Gloss Finishes

Many of the finishing problems discussed in this article will be more noticeable to penmakers and others who use high gloss finishes. That doesn’t mean the problems aren’t there for those who are satisfied that 180-grit is as fine as it gets, it’s just that problems are magnified and more visible when we refine the surfaces to a higher gloss.

The high gloss isn’t for everyone or everything. Sometimes a couple coats of Watco Danish Oil is good enough, but doing so doesn’t mean we can forget about the preparation of the wood surface. Even the Watco oil will look the better for doing it. Torn grain and sanding scratches detract from any finish.

There are two facts to always remember about high gloss finishes. There is nothing that will accent a spectacular piece of wood or grain pattern more than a high gloss finish. There is also nothing that will accent every flaw in the wood, or the finish itself, more than a high gloss finish.

There are two steps to a beautiful high gloss finish – preparation of the wood surface, and refining the finish itself.  First, we have to remove every last trace of torn grain, tool marks, bruising, tiny voids, open grain, and sanding scratches out of the wood surface. Then we have to sand all of the dust and imperfections out of the finish itself, and polish it to the degree of gloss we want.

“It Looks Like Plastic”

Many people, woodturners included, say they don’t like the high gloss finishes because they feel and look like plastic. Words like “feel” and “look” are perceptions and opinions, and we will drive ourselves nuts trying to please everyone. We can’t.

A lacquer, polyurethane, or CA glue finish can look like the plastic because that’s what they are, but it doesn’t have to be that way.  There is a definite connection between the appearance and what we call the “feel” of the finish. When our eye sees a surface that looks like it is covered with a sheet of plastic, our brain tells us that it “feels” like a hard cold sheet of plastic. The opposite is also true. Buff back the polyurethane or CA finish with fine 0000 steel wood to soften the gloss and get rid of those dust bumps; and most people won't be able to tell the difference between the hard finish and bare wood. In other words, when they look the same, the finishes will feel the same.

To test this reaction, I have polished bare wood to a high gloss, and people passed it up and told me that it looked and felt like plastic. Those same people preferred the one with the heavy coat of finish with a softer matte surface because it had the softer and warmer appearance and feel of natural wood.

Scratches

There are two types of scratches – those in the wood under the finish, and those on the finish itself. Both appear to be in the finish; and both are the result of sanding.  The only way to remove either type is to go back to a coarser sanding grit and start over. Preventing them is an easier task.

Scratches in the wood are usually left there from the coarsest grit that was used at the beginning of the sanding schedule.  They could also be there from an oversized particle on the surface of a poor quality sandpaper, or from dust and debris. They are often invisible until they are magnified by the finish; but sometimes we don’t see them until later because of poor lighting.

The simple cure is to make sure all of the scratches from the previous grit are removed before going on to the next grit, always sanding both across and with the wood grain, not skipping any grits, and not using cheap sandpaper.

The sandpaper can be “sized” by dragging it over the sharp edge of a piece of steel. The edge of the lathe bed works, but the back of an old kitchen knife held in a vise is better. This will break off any offending oversized particles. Don’t overdo it. 2 or 3 swipes are enough.

It is always a good plan to progress up through the grits in steps that are no more than 50% greater than the previous grit number. The grits available have already done this for us – 60, 100, 150, 220, 320, 400, and 600.  We could also use the sequence of 80, 120, 180, 240, 320, etc. Don’t skip any of them, unless you like finding scratches in the wood after it has been coated with a finish.

There should be no visible scratches after sanding with 600-grit.  The pencrafter who uses Micro-Mesh to achieve a high gloss should never move on to the grits above 1500MM with the idea that they will remove visible scratches. They won’t. For other woodturners, if you don’t sand to 600, or at least to 400, then you will have to live with scratches. 

I use a bright incandescent light because it casts hard shadows that show up the scratches better than fluorescent lighting, and hand-sand with 600-grit until there aren’t any. I go back a few grits and start over again if I have to. Then I go to 0000-steel wool, leaving a smooth matte surface with no visible scratches. I do this on everything I turn, from pens to salad bowls. Yes, it takes time to do this, but it is well worth the effort because the finish cannot be any better than the surface under it, and I may choose to apply no finish at all. 

Some wood species scratches easier than others. Walnut is one of those. The wood shouldn’t be allowed to get hot because heat makes the scratches worse. Sanding at slow speeds will help. Using wax as a lubricant for the 320 grit and finer will help the most.

The sanding scratches ON the surface of the finish can also be there from the earlier sanding with a coarser grit. The finish doesn’t have to be removed, but it will require going back to a coarser grit and starting over.

Many of the scratches after using Micro-Mesh are the result of dirt and dust particles that got between the surface and the sanding medium. This is one of the hazards of fine grit sanding. Keep the sanding medium clean, and don’t let any airborne dust get under it. Wet sanding with a wax lubricant will help.

Scratches can also be left behind in a surface that was buffed with White Diamond or other fine abrasive compound. It could be shop dirt, or it could be a build-up of hardened buffing compounds in the wheel. Store the wheels where they aren’t exposed to shop dust and dirt, and clean them with the sharp edge of a piece of hardwood before using them with new abrasive compound.

If there are still scratches, try an automotive scratch remover such as Meguiar’s “Swirl Remover”.

Rough, Pebbly, Ridges, or Brush Marks

Bad things happen when the finish dries too fast. Fast drying finishes like shellac and lacquer can be a problem. Ridges happen when too much of a friction polish is applied at one time. Rough and pebbly happens to CA glue that cures too quickly, or when there is still a trace of accelerator on the wood. Pebbly (we call it “orange peel”) happens to a spray finish when the droplets start to dry before they are deposited on the surface of the wood. Any of these can happen to a waterborne when it is allowed to dry on the applicator.

All is not lost if these conditions occur after the finish has cured or dried. Just wait a day, and start sanding. Start at 320-grit, but be prepared to start at 180 or 220 if the surface is really bad, and use water or a wax as a lubricant. The sandpaper doesn’t have to be the wet/dry type. Use whatever is available. The worst that can happen is that the grit particles will come off the paper if the glue holding them on isn’t waterproof. The slurry is a good sanding medium, but clean-up will be messier and the paper can’t be used again.

Ridges in a shellac friction polish are the result of using too much liquid and not getting it hot enough to keep it flowing into a smooth film. Fixing them is easy. Remove the excess shellac with Denatured Alcohol, buff the surface with 0000-steel wool, and start over. Use less of the liquid this time and get it hot. Finger burning hot works best. The same is true for a lacquer friction polish, except that lacquer thinner is the solvent.

Prevention is always the best solution, and this is done by making the solvents evaporate more slowly, or using a finish that has a slower cure.  The longer cure time is easy to do with CA glue on a pen or other small piece of wood, but something has to be added to the other finishes to slow them down. Commercial retarding thinners are available for lacquers. Adding a little Turpentine (Pure Gum Spirits, not a turpentine substitute) will slow down the evaporation of the solvents in shellac or lacquer.  Adding some kerosene to an oil or varnish will lengthen their drying time.  These measures will also reduce brush marks because it will allow the finish more time to seek a uniform level on the surface.

Polyurethane finishes are dust magnets to airborne particles, and they appear as bumps in the new finish. The best cure is applying the finish in cleaner surroundings, or applying the finish in the early morning before the shop dust has been stirred up. The only option is doing a lot of sanding after it is dry.

Waterborne finishes dry fast. Too often they dry too fast. There isn’t much that can be done other than applying a thicker wetter coat, and hoping for the best. Thinning them with water is not a recommended solution. There are retarders available for some of the waterborne finishes, but they should only be used with the same brand because compatibility can be a problem. Applying the finish from a “squeeze bottle” and keeping the surface wet ahead of the applicator will help if it is being applied on the lathe. This works well for pens and bottle-stoppers and the like.  Always read the directions on the can when working with a waterborne finish.

Runs and Sags

These are easy to understand. There was more wet finish on the surface than could support itself until it dried. These are common problems with shellac and lacquer. The obvious solution is to use thinner (in thickness) coats if it is sprayed on, or keeping the piece rotating at a slow speed until the finish dries enough that it won’t run.  An advantage of rotating the piece is that it allows thicker coats to be applied. Runs and sags in a dry finish can be removed with sanding.

Dull, and Cloudy

These can be in spots, large areas, or all over. The reasons for these problems are simple. ALL finishes are permeable to some degree, and some are more permeable than others. Shellac is the most permeable, and epoxies are the least. Everything else is somewhere in between. That means that some liquids or gasses, including water vapor in the atmosphere, can pass through the finish. Even plastics are permeable, and that is why the CO2 in soda pop will eventually leak through the plastic bottle. And, all this time you thought it was leaking through the cap. The only way we could get an impermeable finish would be to coat the wood with glass, a fired ceramic, or metal. Everything else leaks.

"DULL" is caused by oil.

The oil can be natural in the wood or it can be oil that we have added to the wood before we sealed it with a finish. The oil will leak out through the finish over time and kill any gloss that might have been there. This can happen as quickly as overnight, but it will usually take a few days or weeks. The only solution is to remove the finish, and then either remove the oil, or allow it to cure.

We can get temporary relief by removing the oil from the wood surface with Acetone or Lacquer Thinner before applying a finish. Be aware that the natural oils remaining in the wood will still migrate to the surface, penetrate the finish, and kill the gloss. The only difference is that it will take longer. Because of its natural oils, Olive wood is particularly difficult to finish to a high gloss that will stay that way.

If the oil is one that we put on the wood, it has to be totally cured before we can put any other finish on it (see “Adhesion”, below). If it isn’t, the finish will have to be removed, wait a few days to a week or two for the oil to cure, and then try it again. It would have been better to have not used the oil at all.

We could always just leave it alone and accept that the dull finish is OK. Some woods are best left with a dull finish or no finish.

"CLOUDY" is caused by moisture.

Also known as "blush", this is moisture that has condensed under the surface film. This is a common problem when lacquer is sprayed in humid weather conditions, but it can also happen with brushed lacquer and wipe-on finishes like CA glue or shellac based friction polishes. Sometimes it will disappear over time as the moisture migrates through the permeable surface film and evaporates. Sometimes it won't. Usually it is a matter of our not wanting to wait that long.

This moisture can be from within the wood because it wasn't as dry as we thought. Moisture can be driven to the surface from the heat of turning and sanding. We could have added the moisture with the alcohol we used to clean the wood, or it could have come from the humid atmosphere if we used compressed air or another gas (spray can) to carry the finish to the wood. The moisture could also be from an old finishing product that has had considerable exposure to humid air in a partial can that has been opened a lot. Moisture can also be absorbed into the liquid finish if a cold container is opened in a warm room.

Other than removing the finish and starting over, the only solution to a cloudy finish is to wait and see if it disappears with time. This could be a few days to several weeks, depending on how fast the moisture can move through the finish. If that doesn’t work, lacquer or shellac can be softened with thinner, and see if it disappears. The less permeable polyurethane and CA finishes may have to be removed to allow the moisture to evaporate.

Prevention is the best solution to a cloudy finish. Make sure the wood is dry, and let it sit for a couple days between turning and finishing. At least let it cool to room temperature before applying a finish.

White and Black

Pencrafters who use CA glue as a finish can experience everything from cloudy to white when the glue cures too fast. In the extreme case a finish will look like Styrofoam. CA glue gets hot as it cures, and the faster it cures, the hotter it gets. The heat can make CA glue turn white, and extreme heat causes it to bubble and boil into a foam before it hardens.  The solution is simple - stay away from the accelerator and use a slower curing glue. It won’t take more than a minute longer to finish the pen. Think about it – woodturners are the only people who will use glue that is labeled as “instant” and then use an accelerator to make it to cure faster.

"White" spots are a common problem in dark wood, and they are the result of wax, opaque grain fillers, or sanding debris that has collected in the pores of the wood. The solution is to keep the wood clean with an old toothbrush or scrub brush, and avoid waxes and white colored sealers that can build up in the darker wood.

“Black” spots in a light wood can be from the same things that caused the “white” spots in a dark wood, and the solution is the same.

Pencrafters who use a light colored wood often have a problem with a black stain at the ends of their pen barrels. This is from dragging the sanding particles from the steel bushings onto the wood.  Some species are worse than others, and I have found the hard yellow species (Canary, Pau Amarillo, etc.) to be more susceptible to metallic staining. There are several solutions - stay off the bushings when sanding, use plastic bushings, don’t use bushings for sanding, or don’t use wood that has a staining problem. Plastic bushings will have to be made from pen blanks or a material such as Corian. Many pencrafters hold their pen barrels between centers for sanding and finishing. My solution is to not use problem species.

Black streaks can be the result of using the black wet/dry sandpaper in the finer grits, and allowing it to get hot enough to melt the waterproof adhesive that holds the grit particles on the paper. The solution is simple – don’t let it get hot. Slow down the lathe RPM or use a lubricant and keep it wet.

Blotchy or Enhancement?

These describe what we get when the different amounts of finish absorbed by end and flat wood grain causes different amounts of color change in the different parts of the wood. We call it "blotch" when the effect is ugly. We call it "enhancement” when the result is pretty. Since oils impart the most color change in the wood, the oils are a shortcut to both "blotch" and "enhancement".

The best solution to “blotch” is to do everything we can to prevent it from happening. We can either sand the wood to 600-grit, or we can seal it with lacquer or shellac. Either will reduce the absorption of a finish into the end grain areas.

If the “blotch” has already happened, there are only two things we can do. We can remove the finish and sand deep enough into the wood to remove all of the offending color, and then start over by sealing the wood before applying the finish. We can use a commercial sanding sealer, but either shellac or a thinned lacquer will do the same thing. Always sand the sealer back to bare wood so it is IN the wood rather than ON it where it can cause problems with other finishes that are applied over it (see “Adhesion” below). 

We could also put the finished wood in bright sunlight and wait for all of the wood to change color and hope that the blotchy coloring isn't as noticeable. This can work for Cherry in a few days, or Maple if we want to wait longer..

Some wood just gets blotchy from everything we can put on it. Maple can be one of those woods that gets “blotchy” from a coat of wax. I always use a lacquer sealer under an oil finish on Maple.

A particularly troublesome blotch can be the result of burnishing or bruising, the wood surface with either the tuning tool or sandpaper. This happens when the turning tool rubs too hard on the wood and compacts the surface, or when sandpaper gets too hot and burns the wood surface. The result is the same, the finish will not penetrate the area, and the wood will remain a lighter color. The solutions are rounding the corner at the bottom of the tool bevel, learning to slide along the tool bevel with less pressure, sanding without generating heat, and using a finish that imparts less color to the wood.

Adhesion (or lack of it)

You haven’t lived until you have been able to peel the finish off like a sheet of Saran-Wrap. This was a common problem in the early days of polyurethanes finishes, but it can still happen to polyurethane and waterborne finishes if we aren’t careful with them.

There are two primary ways that multiple coats of a finish adheres to itself.  It either melts into the surface under it, or there is a mechanical bond between the finish and the previous coat. It is this mechanical bonding that causes us the most problems. A finish melting into itself would be putting a lacquer over lacquer, or shellac over shellac. The mechanical bond depends on the finish gripping into the scratches in the surface, and this is the primary bond between coats of oil, varnish,  polyurethane, and waterborne finishes. . Adhesion problems are created when the surface is too slick for there to be anything for the finish to grab, or the wood has been contaminated with oil, sweat, moisture, or other chemical substances to prevent bonding.

Adhesion can also be a problem when incompatible finishes are used such as when a waterborne finish is applied over an oil that hasn’t fully cured. The waterborne has a better chance of sticking if the oil was given sufficient time to cure, but that can take a long time when atmospheric conditions are at their worst.

The solution is simple. Be clean and don't touch the wood with our dirty or sweaty fingers. Dripping sweat is always a problem when it is hot and humid. Always remove the gloss from the previous coat by abrading it with steel wool or sandpaper. And, never use a silicone product such as car wax in the shop because there is nothing that will repel a finisher faster, especially if the finish is lacquer or CA glue.

The incompatible finishes can also be separated with a barrier coat that is compatible with both. Shellac is the universal finish – it sticks to everything and everything sticks to it. The best solution, however, is to not use incompatible finishes like a lacquer or a waterborne finish over an oil.

If the oil is IN the wood as a natural part of the wood, it will have to be removed on the surface as best we can with a solvent before applying a finish, but be aware that the remaining oil will migrate up to the surface with time and can still break the bond between the wood and the finish.

The Finish Won’t Cure

This is a common complaint about oil finishes when the weather is cooler in the winter, but it can also happen when the temperatures and relative humidity are both close to 100.  The ideal conditions for an oil finish are 70ºF temperature and 70% relative humidity. It is too cold to use an oil finish when we have to wear a coat in the shop, and it is too warm when there is sweat dripping off our nose. Both conditions are worse when it is raining outside.

An oil finish cures by the chemical reaction of polymerization where the smaller individual molecules of the liquid link together to make the larger molecules of the solid. The catalyst for this reaction is the oxygen in the air and the heat created by their joining. Most commercial finishes have “drying agents” added to them that insure there is oxygen available for the reaction. The thinner is there to provide a pool for the molecules to move around in so they can find each other. Too cold and there is no heat for the reaction to take place, and if the thinner does evaporate, it leaves a sticky oil behind. Too hot and the thinner evaporates before the molecules can get together, and the finish remains as a soft jell that might approach becoming a hard surface, but never quite gets there.

We can adjust for the hot weather by using thinners with a slower evaporation rate. Thinning an oil finish with kerosene will give it more time. We can adjust for cold weather by adding oxidizers such as Japan Drier. The best solution is to always obey the 70ºF/70% rule and not going more than 10 in either direction.  We can provide an artificial environment with heat and air conditioning, but make sure the wood is at the same temperature as the atmosphere around it.

Cracking (in the finish)

Cracking can be the result from using incompatible finishes. This cracking can be intentional for decorative effect, such as when we put a nitrocellulose lacquer over an oil paint. But, it can also happen when we mix different brands of the same finish because the thinners or the additives are incompatible. The solution is to not switch finishes or brands in the middle of multiple coats of any finishing schedule.

Most cracking is unintentional, and a surprise when we find it. This cracking happens when the finish shrinks and the wood doesn’t, or the wood expands and the finish doesn’t. It can happen immediately if the wood is finished when it is at a moisture content that is less than equilibrium with the surrounding atmosphere, and the finish doesn’t have the flexibility to move with the wood. We can prevent a lot of cracking in finishes by letting the wood cool to room temperature and reach equilibrium before putting a finish on it. Waiting a couple days before finishing a newly turned and sanded piece of wood is a good practice.

Cracking can happen long after the finish was applied as the wood moves with seasonal changes in moisture content and the finish has become brittle with age. This is a common problem with Nitrocellulose Lacquers, and can also happen to an older Polyurethane finish.

There are several things we can do. Don’t use an accelerator with a CA finish because it makes it more brittle. There are plasticizers available for all types of lacquer, but the old-fashioned method of adding 1 to no more than 2 tablespoons of Castor Oil to a quart of lacquer still works. The best solution is to not use thick lacquer or polyurethane finishes on a wood species that moves a lot with seasonal changes in moisture content.

Cracking is rare with oils and varnishes because they are more flexible and can move with the wood, but they too have their limits. Cracking is common in very old varnish finishes.

Prevention is the best cure. Let wet wood dry before using it, let dry wood acclimate to the shop before turning it, and always let the wood cool to room temperature before finishing it.

 

Thermal expansion can be a problem. CA and polyurethane finishes can crack when they are exposed to low temperatures because they shrink more than the wood and lose what little elasticity they have with the lower temperatures. Shipping turned items with these finishes can be a problem during the winter.

Cracking (in the wood)

The wood cracks after it is finished when the same conditions that caused the finish to crack are worse, and there is more movement than the wood can tolerate. Hairline cracks could have already been there and weren’t visible until they grew after the wood was turned and finished. Then there are woods like Snakewood that crack for no other reason than because it wants to, and there is little to nothing we can do about it.

Wrinkled and Puckered

These are caused by the opposite conditions from those that made the finish crack. Most finishes can’t be compressed very well when the wood shrinks. Shrinkage of the wood is usually the result of it not being as dry as we thought it was. It may be dry enough for the finish to adhere, but still be several percentage points above its equilibrium moisture content.

Some exotic woods have a large coefficient of thermal expansion. This means they expand a lot as they become warm from turning and sanding. If they are finished while they are still warm, the wood will shrink more than the finish, and that extra finish has to go somewhere. Finishes on these species can also pucker when they are exposed to low temperatures.

Just as with cracking, wrinkles in the surface can also be from incompatible finishes or products. It can be a “decorative effect” when it is done on purpose, and a disaster when it isn’t. Again, don’t change finishes or product brands in the middle of several coats. 

Thick and Thin

These are generally geometric problems because some finishes want to pull away from a sharp outside corner, and make a thick fillet in an inside corner. This makes the finish wear away faster on the outside corners, and pull away from the wood or crack on the inside ones. These conditions are most noticeable with lacquer that is applied with a spray, but it can be a problem with all finishes to some degree. Again, the solution is simple – avoid sharp corners.

Avoiding sharp corners at the ends of a pen barrel is impossible to do and still have any kind of a matching fit with the metal fittings. If a thin or rounded finish is a problem, carry the finish out over the bushings at the ends of the pen barrel, and then cut through the finish at the joint with a skew or sharp knife before removing it from the mandrel. Don’t break the bushing loose from the barrel because a jagged break may cross over to the end of the barrel.

Conclusion

There are few problems with finishing wood that can’t be resolved with patience and sandpaper.  Don’t get in a hurry. Allow the wood time to dry before applying a finish. Allow the wood and everything else to cool (or warm up) to room temperature before applying a finish. Don’t change finishes or brands in the middle of finishing a piece. And, don’t use strange chemicals and elixirs that promise to make finishing faster. When all else fails, start sanding.

Finish Gone Wrong??

By Russ Fairfield

Common finishing problems. How to fix them, how to keep them from happening again

This is the draft copy of an article that was published in Woodturning Design, Issue 22, Summer 2009.

This page was last updated: December 10, 2011