How to fix flaws in film finishes
You're not alone if you examine a newly finished project from every angle for drips, runs, and sags. On a good day, you sigh with relief; on a bad day, you groan with disappointment.
Now you see drips; now you don't
Avoiding problems in film finishes proves much easier than fixing them—even with these techniques—so plan to succeed with these preparation basics:
- You can't avoid what you can't see, so work under strong reflected light, as shown below. Move the light to reflect off each surface as you finish it.
- Apply finishes to horizontal surfaces whenever possible—even if that means laying a project on its side to finish it. That still takes less time than removing runs and starting over.
- Thin finishes up to 10 percent to help brush marks level off, but don't over-thin a finish unless you plan to wipe it on. More brushed-on coats mean more opportunities for mistakes.
- Avoid sanding sealers, especially over stained wood. Instead, apply a full-strength or slightly thinned coat of the same finish you'll use for the remaining coats. You're less likely to sand through these than a thin layer of sanding sealer while removing any imperfections.
Don't rush the brush
Old-time painters didn't talk about brushing on a finish; they laid it on using the brush as a carrier.
Prepare the brush by slapping the dry bristles against your palm to reveal loose ones. Then submerge the bristles in solvent suitable for the finish you'll use—mineral spirits for oil-based polyurethane, for example. Squeeze the bristles with a clean towel to remove the excess solvent. This rinses debris from the brush and helps make the brush easier to clean later.
Pour finish into a separate container to avoid contaminating the original can with your brush. Dip the bristles about half-way into the finish, and draw off the excess against the container rim. Apply finish near one edge about 4" from an end, as shown below. Then brush back toward the end without forcing drips of finish off the wood.
Squeeze out excess finish
Lay finish thick enough to cover the wood and level off, but not so thick that it pools on the surface. If you've applied too much finish, immediately unload the bristles into a dry container, as shown in photo, below, and brush back over the surface to pick up the surplus.
As soon as you've applied finish to an area, help remove bubbles by "tipping off" the finish using the bristle tips drawn lightly across the surface, as shown in photo, below This will eliminate most problems before they start and it helps level off the surface.
Fixes for small flaws
Despite your best efforts, stray drops of finish can harden into a bump, or an overflowing brush can leave a sagging lump on the underside of an edge. Try these fixes only after the finish dries at least a day or two to avoid marring a soft surface. Because film finishes dry at the surface first, what feels dry on top could cover a gummy layer underneath.
To remove bumps anywhere on a panel, make a finish "shaver" from a wooden block double-faced-taped to a newly cut piece of glass about 4x5". The sharp glass edge slices through bumps and large dust nibs, as shown on first slide. But the flat face doesn't dig into or scratch the surrounding finish.
For bumps near an edge, another option is to shave them off using a sharp, burr-free chisel. Lay the chisel flat on the surface bevel-side up, and guide the edge against the bump at an angle, as shown below. Allow the newly exposed area to harden overnight, sand it smooth with 320 grit, and apply another coat.
For gaps in the finish—usually near the edges—scuff sand the finish around the gap with 320-grit sandpaper without touching the bare area. Then apply finish to the bare wood, feathering the wet edge against the edge of the cured coat. Allow the finish to dry one day before lightly sanding the surface smooth and applying another full coat.
Drastic action for big flaws
Because polyurethane hardens slowly, there's always a "whoops window" when a machine may kick dust into the air, outdoor breezes may deposit debris on a fresh finish, or a project part may tumble finish-side-down to the floor. If you catch the problem while the surface remains tacky or wet, remove the finish immediately with the appropriate solvent, and try again.
If you discover a major problem after the finish skins over or hardens, then it's time to start over. For flat, unstained surfaces, simply sand off the problem or remove it using a cabinet scraper. If you opt for sanding, check your abrasive frequently for finish build-up and clogging that can damage the wood.
On rounded corners or hard-to-sand areas, apply a furniture stripper to remove the finish while it's still curing. Following the manufacturer's directions, carefully apply stripper to only the flawed surface. Allow the finish to dissolve, and scrape it from flat surfaces using a plastic or dull metal putty knife, as shown below. To remove stripper from crevices, use a sharp sliver of wood. Wipe the remaining stripper away with a coarse burlap cloth.
After using a methylene chloride product, such as the one we used here, rinse the surface with mineral spirits or naphtha to remove leftover stripper and break up waxy stripper residue. By scraping carefully, you can lightly sand the stripped surface using 220-grit abrasive to knock down any raised grain. Then, older and wiser, you're ready to apply a fresh coat of finish.