How To Repair A Dry Spot In Polyurethane Finish

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This article is from Issue 49 of Woodcraft Magazine.

Recovery tips for when things go awry

When you finish and refinish furniture for a living like I do, you quickly discover that it's unusual for a finishing job to go flawlessly. From the initial sanding to the final waxing, problems happen, and good finishers learn to react to them calmly as they arise instead of panicking.

Finishing problems run the gamut from overlooked glue spots and sanding scratches to drips, sags, and trapped debris in the finish. I'll address some of the more common problems that you'll face at the beginning, middle, or end of the process, and show you how to fix them on the fly.

Remove a glue spot on a stained surface by wet-sanding it with sandpaper dunked in the stain.

A scraper will remove glue spots or scratches after thoroughly wiping away the finish and letting the surface dry.

Glue spots and sanding scratches

Glue spots show up as light areas under a stain or a finish (See photo at left), while coarse sanding scratches darken. It's best to prevent these problems altogether by performing a thorough inspection of a surface before applying finish. Sand it, blow off the dust, and wipe the project down with naphtha (for solvent-based finishes) or denatured alcohol (for water-based products). The wet solvent highlights glue spots and sanding scratches.

If any glue spots or scratches elude the pre-finish inspection, you'll have to deal with them during the finish process. The fix will depend on whether you're staining first or simply applying a clear finish.

When staining, wet-sand any surprise glue spots or sanding scratches immediately using the same grit sandpaper you used as the final grit on the bare wood, dunking the paper into the stain first (Photo A).

In certain situations you can use a sharp chisel to pare away glue. This will leave a white spot that can be touched up by scrubbing the area with a stain-soaked abrasive pad to blend in the stain. When wet-sanding scratches, avoid creating a dip by feathering out the area as described in the sidebar at right.

When you encounter a blemish in a clear finish, wipe off as much of the finish as you can from the affected area with a clean rag, and then follow up with the appropriate thinner (mineral spirits for oil-based varnish, for example). Let the surface dry, and then use 150-grit sandpaper or a scraper to remove the glue spots or scratches (Photo B). Make sure to feather out the area as you work to avoid creating a low spot that a finish will accentuate. When sanding, work through the same sequence of grits you used originally.

Feathering Out

When removing errant glue, sanding scratches, or other blemishes, it's important to avoid creating a small divot, which a finish will accentuate. The trick is to "feather out" the corrected area by sanding outward in all directions to create a larger, but much less noticeable depression. Begin with the finest-grit sandpaper that will smooth away the defect, wrapping the paper around a cork-faced backer block. First, sand diagonally in both directions, then across the grain, and finally with the grain. Decrease your sanding pressure as you move away from the center of the depression. For faster results, you can use a scraper taking the same directional approach.

Wiping naphtha onto the sand-through on this door clearly reveals that the underlying stained surface has been compromised.

Fixing sand-throughs from scuff sanding

When scuff-sanding between coats of finish, you'll occasionally sand through the finish. This can also happen when rubbing out a finish. If the damage goes deep, you may have to reapply color (if you stained), as well as finish. To assess the damaged area, wipe it with naphtha (Photo C). If the rub-through seems to disappear, matching the rest of the surface, you need only replace the finish, applying it with a small artist's brush.

Use the side of the bristles rather than the tip to apply stain to an edge that's been sanded through.

If the wipe-down reveals an area of bare wood, you'll have to replace both color and finish. Mixing alcohol-soluble dyes with shellac is a quick and easy way to apply color (Photo D). After replacing the color, coat it with a clear finish to protect it.

A scraper will quickly remove dried blobs of Danish oil that seep to the surface of open-pored woods.

Danish oil bleeding from pores

When using a "Danish oil" or other type of highly thinned, oil-based, wipe-on finish, you may experience oil seeping from the pores of open-grained woods like oak and ash after the surface has been wiped off. The problem is caused by surface tension disparity, in which the difference between the state of the drying oil at the surface and the wet oil in a pore causes the latter to "climb" out of the pore, creating a blob around it.

To prevent the problem, disregard the "flood-on-and-wipe-off" product directions when finishing open-pored woods. Instead, initially wipe on just enough oil to wet the wood, wipe off the excess, and allow this first coat to dry for six hours or so. Repeat this twice more, and then you can flood the finish on if you like.

If you do experience bleeding from flooding on too much oil, you have two options: You can either continue to wipe away the wet seepage until it stops, or you can let the blobs dry, and scrape or sand them away (Photo E). When the bleeding stops and the surface is smooth and dry, apply one or two more thin coats to restore an even luster.

Ridging and cratering are the telltale signs of silicone contamination. The only solution is to start over from scratch.

Fish-eye

Fish-eyes are craters or ridges that form in a finish as a result of contamination on the wood surface (Photo F). They can appear when finishing fresh wood that's been exposed to silicone or other oily contaminants, but most often show up when refinishing projects that have been improperly stripped. The problem is often that the stripper didn't thoroughly remove all of the residual wax and silicone introduced through the use of furniture polish over the years.

After wiping away a fish-eyed finish, scrub away the afflicting contamination using TSP and a synthetic pad.

Unfortunately, the only fix is to start over after removing the contamination. If possible wipe away the failed finish while it's wet, using the appropriate solvent. Follow up with a wipe-down of denatured alcohol, and allow the surface to dry. Mix TSP cleaner with water, according to the product instructions, and use it along with a green or maroon synthetic pad to thoroughly scrub the surface (Photo G).

Remove any residue with a clean cloth, let the wood dry, and smooth the surface with 220-grit sandpaper. Then apply a coat of 2-lb. dewaxed shellac to the area, which will isolate any remaining silicone. When the shellac is dry, reapply your finish.

A dental pick is a great tool for cleanly lifting bits of debris from a finish.

Removing trapped dust, insects, or other debris

Any professional finisher can tell you stories of things falling into a wet finish, including dust, bugs, hair, and peeling ceiling paint. My personal favorite was the largest moth I've ever seen burrowing a trail completely across a freshly lacquered dining table in an attempt to extricate itself. (That required a complete redo.)

If a finish is still wet, pick out the debris using a toothpick or–better yet–a dental pick or needle (Photo H). A still-wet finish will typically flow back on itself, but it can help to apply a bit more finish on top of the area. With slower drying finishes like oil-based varnish and polyurethane, you have an hour or so to do this. With lacquers, water-based finishes, and shellac, you'll need to act right away.

If a finish has started to set up, let it dry completely, and then sand away the offending material. In this case, I dry-sand with 400-grit "P grade" sandpaper. Wet-sanding can be dangerous in this case because the lubricant can mimic the finish, preventing you from noticing that you're cutting through it. As before, feather out the area to help hide the repair. Then apply another coat of finish.

Scrub with a stain-soaked abrasive pad to blend in stain drips that land on an already stained surface.

Drips, sags, and runs

The easiest way to fix finishing flubs is to adopt a strategy that prevents them from happening in the first place. When staining, I try to work from the "bottom up" on a project so that any errant drops of stain land on something that's already stained. If you drip stain–particularly fast-absorbing dye stain–on bare wood and don't tend to it right away, it will show as a dark "double-stained" spot later.

If you can, disassemble the project into manageable parts, and stain them separately. If you do drip stain onto bare wood, finish up the area in process (to avoid lap marks), and then immediately apply stain to the bare area that suffered drips. If the drips resist blending into the stain, try scrubbing the area with a piece of gray synthetic pad soaked with stain (Photo I).

Use a chisel with a slicing motion to level hardened drips to the adjacent surface.

Even with solid preventative measures, an overloaded rag or heavy-handed application can result in a few drips, runs or sags. How you deal with these problems depends upon whether you detect them when the finish is wet or when it's dry. With a wet finish on an unstained project, you can just wipe away the drip or run with your finger, and then reapply finish. If the project is stained, wipe the drip very lightly to prevent lifting the stain.

If you notice a drip or run in a finish that has started to set up, it's best to let it dry before removing it. Once it's dried, don't just try flattening it with sandpaper, which risks sanding through the adjacent area. Instead, slice the drip level with a razor-sharp chisel (Photo J). (When possible, orient the back of the chisel flat against the surface for good registration.) Slicing in a slight arc cuts much more cleanly than just pushing the tool forward. Follow up by sanding the area very lightly, and then apply more finish.

Dealing with a finish that won't dry

If an applied finish remains persistently tacky beyond its stated dry time, the problem is probably with the wood, not the finish. Rosewood, teak, and aromatic red cedar all contain substances that can impede drying.

Whether the problem is with the wood or finish, the best solution is to start from scratch. Remove the finish with mineral spirits and wipe it clean with fresh lacquer thinner or denatured alcohol. Apply a coat of 2-lb.-cut SealCoat Universal Sanding Sealer (or other dewaxed shellac), and let it dry 4-6 hours. After that, apply either lacquer or an oil-based varnish.

Bubbles most commonly appear when brushing on water-based products and oil-based varnishes.

Bubbles in finish

Bubbles appearing in a coat of finish is one of the most exasperating problems in finishing (Photo K). Fortunately, it doesn't happen with all products–mostly with water-based products and oil-based varnishes. It's usually not a problem with shellac and solvent-based lacquers. Bubbling is a result of the agitation created by the bristles when brushing. (Spraying or wiping on the same finish is usually bubble-free.)

If you experience bubbles in your finish, let them dry completely. Then sand the finish level with 400-grit sandpaper and apply another coat, paying attention to the following guidelines:

Water-based finishes contain chemicals that are designed to help mix the finish's components. These detergent-like additives tend to foam when agitated. To counteract this, manufacturers add "de-foamers" that help induce bubbles to break. So be patient when you see them appear in your wet coat, and give them time to break. If they remain a problem, try stirring the product more thoroughly. You can also add 5 to 10 percent water, and perhaps try using a better quality brush.

Oil-based finishes usually bubble as a result of agitation by the brush. To minimize this, use a good quality natural-bristle brush, and don't drag the bristles across the lip of the container to remove excess finish. Instead, press them against the inside wall of the container, and try not to work the brush too vigorously as you apply finish. As with water-based finishes, give the bubbles a few minutes to pop by themselves. If they don't, try thinning the finish 10 to 15 percent with mineral spirits, which should help.

Lacquer "blushes," or turns white, when water vapor is trapped in the finish on warm, humid days.

Blushing

Applying a fast-drying solvent-based finish like shellac or lacquer on a warm, humid day can cause the finish to "blush," or whiten, as it dries (Photo L).

This happens as the fast-evaporating solvents in the finish cool the air adjacent to the finish, causing water vapor to form and mix with the finish, turning it white.

Professionals prevent blushing on warm, humid days by using special finish additives. Unfortunately, these aren't commonly available, so the nonprofessional has to take a different approach. If your finish blushes, your best bet is to let it dry, sand it, and then apply another coat after the humidity drops back to 50-65 percent. It also helps to add about 10 percent thinner to the first one or two coats. If blushing continues, you'll have to slow down the drying enough to allow the water to evaporate from the finish. To do this, add about 10 percent isopropyl or butyl alcohol to shellac. For lacquer, use lacquer retarder.

About Our Author

Jeff Jewitt is a professional finisher who has been teaching and writing on the subject for over 17 years. Owner of Homestead Finishing Products, he has developed finishing products sold worldwide under the Homestead brand name.