Pre-stain conditioner and washcoat

Overview

Some woods take stain unevenly: soft earlywood and swirly grain soak up pigment and go dark, while denser areas stay pale. Popular Woodworking's contributors name pine, fir, birch, maple, poplar and cherry as the usual offenders. A pre-stain conditioner partly seals the absorbent areas so they take less stain. Flexner defines it as a washcoat: finish thinned to 10% solids or less. Shop-made versions cost less and, per Southwick, can be tuned to the wood.

When to use

• Before oil or water-based stain on blotch-prone woods (Eastern white pine, Knotty pine, Hard maple, Yellow birch).
• Not needed with a clear film or oil on its own, though cherry can blotch even under clear finish (Flexner).
• Not a cure for sanding: Flexner says sanding finer lowers the colour a stain gives, but does not reduce blotching (Sanding schedule).
• Alternatives: gel stain, which sits on the surface and is the usual advice for pine (Gel stain), or a different colouring method; see Finishing resinous and blotchy woods.

Materials and tools

A commercial conditioner (see products) or one of the washcoats below; a brush or rag; a clean, dry cloth; and 400 grit paper for water-based follow-ups.

Washcoat recipes (Flexner). Lacquer with at least 1.5 parts thinner; varnish with at least 2 parts thinner; or shellac at a 3/4 lb cut or less (Shellac, Dewaxed shellac, Nitrocellulose lacquer). Commercial wood conditioner is itself a varnish washcoat. Southwick's shop-made conditioner is clear gel varnish.

Steps

1. Sand, remove dust, and wet a corner with mineral spirits or alcohol: blotch-prone patches show before you commit (Southwick). Skip mineral spirits ahead of water-based work.
2. Apply the conditioner or washcoat in an even coat and wipe off the excess per the label. For Minwax oil-based conditioner the label says penetrate 5 to 15 minutes, wipe, and stain within 2 hours.
3. Let it dry far longer than the can says. Flexner says conditioner must dry thoroughly, 6 to 8 hours or overnight, contrary to the label's 15 minutes to 2 hours. Sources disagree here: the Minwax label is the opposite approach (stain within 2 hours), so test on scrap.
4. For water-based stain or dye over a varnish-type controller, dampen with water and sand lightly with 400 or finer (Southwick).
5. Apply the stain (Oil-based pigment stain, Water-based pigment stain, Water-soluble dye stain, Alcohol and oil-soluble dye stain).

Southwick's gel-varnish method. Commercial conditioner, used per the can, seals only about 10 to 15% of the wood; he aims for about 20 to 40%. Method 1: brush clear gel varnish, wipe within about 5 minutes, then oil-stain at once (about 30 to 40%). Method 2: 1 part gel varnish to 4 parts mineral spirits, rub off after 5 to 10 minutes, dry overnight (about 20 to 30%). Stronger mixes seal more: 2:1 about 40 to 50%, 1:1 about 50 to 60%, raw gel dried overnight about 60 to 75%.

Common problems

• Still blotchy. Too little seal, too short a dry, or black pigment in the stain (Southwick: it contributes to blotching). Seal more or switch to gel stain.
• Stain too pale. Over-sealed: the higher the percentage of seal, the less stain the wood takes (Southwick's figures above). Use a weaker mix.
• Pine. Tim Johnson's process: water-based conditioner twice, then dye, then dewaxed shellac, then glaze. Grain reversal (earlywood colouring more than latewood) can still occur (Eastern white pine).
• Cherry. Blotches even under clear finish; factories use glazes and toners.

Safety and environment

Oil-based conditioners are flammable; Minwax's label says rags, steel wool and sanding residue soaked with it can ignite spontaneously: place them in a sealed, water-filled metal container (Oily rag safety). Ventilate.

Related

• Gel stain, Oil-based pigment stain, Water-based pigment stain, Water-soluble dye stain, Alcohol and oil-soluble dye stain
• Sanding sealer (and vinyl sealer), Grain filler (pore filler)
• Finishing resinous and blotchy woods, Sanding schedule
• Wipe-on application