Build coats and sanding between

Overview

A film finish protects by thickness, but each product has a coat count and recoat window set by its chemistry. Dry film per coat is small: Minwax's data sheet gives 0.25 mil per coat of Wipe-On Poly (29 to 31% solids), which is why its label says to apply several coats. Thicker is not always better: Bob Flexner says thinner coatings usually last longer, and four or more coats protect better but crack and craze more as the finish ages.

When to use

• Every film finish. Not needed for pure oils and hardwax oils, where you wipe off the excess and work in very thin coats; see Hardwax oil.
• Use the maker's data sheet for the exact numbers; those below are illustrations, and the data sheet governs.

Materials and tools

• 320 or 400 grit paper by hand for dust nibs (Flexner); 400 then 600 wet for the build coats in Woodcraft's schedule; 220 grit after 2 to 3 hours on Minwax Wipe-On Poly.
• A gray non-woven pad when you want a gentler scuff.
• Vacuum or tack cloth; see Dust removal and tack cloths.

Steps

1. Read the data sheet: touch time, recoat time, and number of coats, noting the test conditions. Minwax's times are for 77F and 50% relative humidity and stretch with cold, humidity, still air or heavy application.
2. Examples from makers (not a ranking):
   • Minwax Wipe-On Poly: recoat 2 h, 2 coats recommended, final 24 h before light use.
   • General Finishes Arm-R-Seal (oil-based urethane): touch 12 h or more, recoat 24 h or more, 3 to 4 coats, cure 30 days; thin coats by rag.
   • Minwax Polycrylic Max (water-based): touch 1 h, recoat 2 h, light use 24 h, no steel wool; the maker's sheet says no sanding between coats is needed.
3. Wiping varnish (Pekovich): coat 1 flooded and wiped off, coat 2 left, level with a wet-sanded third coat using 400 grit, then four to six thin coats. Flexner sands overnight-dried coats with 320 or 400 grit by hand, removes dust and recoats.
4. Woodcraft's four-day schedule for a durable film: thinned seal coat and wipe-off on day 1; wet-sand 400 and apply the first full-strength coat on day 2; wet-sand 600 and a second full coat on day 3; then cure at least 3 days before rubbing out with 0000 steel wool.
5. Sand only to level dust nibs and hardened bubbles, then wipe off every trace of slurry or dust. Sanding through a thin coat on edges is the main risk, so use a gentle pressure at corners.
6. Stop when the film looks and feels right; extra coats add cracking risk (Flexner) and can be rubbed out; see Rubbing out a finish.

Common problems

• Soft, gummy sandpaper: coat not dry or too thick; wait, or see Curing conditions and time-to-use.
• Lifting or wrinkling when recoating: recoat window missed or an incompatible product; see Troubleshooting finish defects.
• Visible witness lines: sanding through between coats on finishes whose coats don't fuse (Woodcraft).
• Grain raise on water-based: sand lightly after the first coat; see Raising the grain.

Safety and environment

Wet-sanding slurry and solvent rags: see Oily rag safety. Minwax warns that rags soaked with its wipe-on may spontaneously ignite and should go into a sealed, water-filled metal container.

Related

• Wipe-on application, Brush application, Spraying with HVLP: application methods.
• Tabletop finish schedules: complete schedules with times.
• Sanding schedule: bare-wood grits.
• Sanding sealer (and vinyl sealer): build-promoting first coat.