Glue squeeze-out and common finish failures

Overview

Glue that has soaked into bare wood seals it, so stain and some finishes cannot enter and the spot stays lighter. Flexner's "Prevent, Remove and Disguise Glue Splotches" says glue splotching usually shows up as lighter areas around joints or from fingerprints, especially under a stain. The cheapest fix is at assembly. Failing that, find the glue before you finish, remove it, and sand the area to the same grit as the rest. This entry covers that, then summarises his baker's dozen of finish application failures, because glue is one of them and several others relate to preparation.

When to use

• After glue-up and before the final sanding (Sanding schedule).
• When pale patches show after staining.
• When a finish misbehaves and you want to know whether the cause was the wood, the preparation or the application.

Materials and tools

Water or mineral spirits for finding glue; a scraper, sandpaper and a soft brass-bristle brush; vinegar, hot water, acetone, lacquer thinner, toluene or xylene as solvents for the glue type; clean cloths. Safety notes below.

Steps

1. Prevent. Flexner's first advice is to put less glue in the joints and to quickly wash off glue seepage and sand smooth. The cabinet-grade finishing guide agrees: wash off wet glue, or lift it with a putty knife when it begins to set.
2. Find the glue. Wet the whole surface with water or paint thinner. It soaks deeper into unsealed wood and leaves glue-sealed areas lighter. Water raises the grain, so let it dry and sand again; Flexner calls this "sponging or dewhiskering" and says it is wise anyway if you will use a water-containing stain or finish (Raising the grain). Commercial additives to white and yellow glues make them show under black light.
3. Remove it by scraping, sanding or solvent. Vinegar or water (hot works best) breaks down white, yellow and hide glues. Acetone, toluene, xylene or lacquer thinner soften white and yellow glues. Acetone or lacquer thinner dissolve contact cement and thin epoxy and polyurethane glue until it cures; after that you need a methylene-chloride paint stripper, sanding or scraping.
4. Scrub along the grain with a brass-wire brush to pull glue out of the pores, then wipe dry.
5. Re-sand. Use the same grit as elsewhere; scraped or sanded areas get finished with the finest grit used elsewhere, so scratch patterns match and colour differences do not show under stain (Dust removal and tack cloths).
6. If you find it after staining, sand or scrape through the stain and restain, or disguise it after a finish coat. Wet-sanding with stain on the paper blends; if the part is lighter, wet-sand again with a coarser paper, because stain lubricates the paper. Otherwise strip the stain, resand and restain.
7. Disguising after a finish coat. Draw in grain with pigment in shellac or padding lacquer using a fine artist's brush, soften with #0000 steel wool, add a thin barrier coat, colour between the lines, then continue finishing. Flexner notes this does not work well on oil finishes because there is no film build (Shellac, Nitrocellulose lacquer).

Common problems

Flexner's "baker's dozen" lists these (avoid / fix):

• Sticky finish. Warmer room; on oily woods wipe with naphtha or acetone before finishing or apply a shellac sealer coat (Degreasing oily woods). Fix with heat or strip.
• Fish eye (craters from silicone in polish). Wash with a solvent, ammonia or trisodium phosphate, seal with shellac, or add a fish-eye eliminator; fix by sanding level.
• Pinholes over open pores in sprayed lacquer. Dust coats, filled pores and avoiding wet early coats (Grain filler (pore filler), Spraying with HVLP).
• Orange peel. Thin the finish more, raise the air pressure, adjust speed and distance, then sand level.
• Blushing (milky lacquer or shellac in humid weather). Wait for a drier day or add retarder; it often fades overnight.
• Brush marks and bubbles in varnish. Thin enough to flow, work cooler, tip off; sand level and recoat (Oil-based polyurethane).
• Stain streaks. Use slower-drying stain, work in smaller areas or get a second person to wipe (Water-based pigment stain).
• Bleeding oil. Keep wiping every half hour or so until pores stop weeping (Wipe-on application).

Safety and environment

Acetone, lacquer thinner, toluene and xylene are flammable and harmful: ventilate, wear gloves and a respirator, and keep away from ignition (check each product's safety data sheet; see Oily rag safety for rags). Flexner lists vinegar and hot water as the first options for white, yellow and hide glue.

Related

• Sanding schedule, Raising the grain, Repairing defects before finish
• Finishing veneers, Degreasing oily woods, Spraying with HVLP
• Wood moisture and acclimation before finishing, Rubbing out a finish