Brush application

Overview

Brushing is the fastest way to build a thick, protective film without spray equipment. Woodcraft's authors note that it builds much faster than wipe-ons and that most failures come from fear of drips, streaks and brush marks, which are avoidable with a few habits: a good brush, a clean dust-free room, light, and restraint. Oil-based and water-based varnishes brush differently; water-based dries fast and punishes second passes.

When to use

• Varnish or Water-based polyurethane on tables, doors and cabinets where you want build in fewer coats than wiping gives.
• Flat surfaces especially; verticals need thin coats and care.
• Not for fast-flashing lacquers, which belong on a gun; see Spraying with HVLP. For the easiest, most forgiving option see Wipe-on application.

Materials and tools

• Oil-based varnish: a natural (china bristle) brush, or a synthetic nylon/polyester with flagged tips. Water-based: synthetic only, because water makes natural bristles splay (Woodcraft; Polyvine).
• Chisel-edge brush for general work, sash brush for corners; avoid square-edged. Woodcraft puts a good 2 in brush at $25 to $80 and says cheap ones shed and spread poorly.
• Mineral spirits (oil-based) or water (water-based) for priming and cleaning.
• A strong reflective light, and triangular points or screw boards to lift the work.

Steps

1. Sand to 220, remove the dust, and work at 65 to 80F with moderate humidity; cold slows drying and heat dries before the finish levels (Woodcraft).
2. Plan the sequence: undersides and interiors first, the top last, with each surface horizontal when possible so it levels (Polyvine).
3. Prime the brush in the matching solvent and shake out the excess.
4. Stir, never shake; shaking adds bubbles. Thin the first oil-based coat 10 to 50% with mineral spirits; check the label before thinning water-based (Woodcraft). See Thinning and viscosity.
5. Load the tip only at edges; load fully on broad areas but not enough to sag. Start on an unfinished area and end on wet edge, with long straight strokes along the grain (Polyvine).
6. Tip off once: drag the bristle tips lightly over the wet surface, then stop. Do not back-brush once it begins to set (Woodcraft; Polyvine). Wipe drips with a rag wrapped round a finger.
7. Dry fully, sand lightly, dust off, repeat; see Build coats and sanding between. Wet grain raising on water-based coats: dampen, dry, re-sand and it raises far less afterwards (Polyvine).

Common problems

• Brush marks: finish too thick or re-brushed while setting. Thin slightly, work faster, tip off once.
• Bubbles: shaken can or overloaded brush; brush them out while wet.
• Runs and sags: heavy load; check often in reflected light. See Troubleshooting finish defects.
• Stray bristles: low-quality brush.
• Lost wet edge in warm rooms with water-based finish (Polyvine). See Thinning and viscosity for retarders.

Safety and environment

Ventilate, keep solvents away from ignition, and spread solvent-soaked rags flat; see Oily rag safety. Clean water-based brushes promptly, because Polyvine says dry varnish builds up in the stock within minutes.

Related

• Oil-based polyurethane, Water-based polyurethane, Alkyd varnish: brushing finishes.
• Curing conditions and time-to-use, Rubbing out a finish: after the last coat.
• Dust removal and tack cloths: before each coat.