Grain filler (pore filler)

Overview

Large-pored woods leave a pitted surface under a thin film. You can fill the pores with the finish itself, or pack them with a paste filler. Flexner (Popular Woodworking) describes filler as silica in an oil binder with a little thinner; Michael Dresdner describes it as a thick mixture of inert solids (ground sand, talc, pumice or wood flour) and a resin binder. Only some woods need it, and it is a taste choice: filled finishes look flat and glassy, unfilled ones show the grain.

When to use

The Wood Handbook puts large vessels in ash, hickory, oak and walnut, and small vessels in aspen, cherry and maple, and recommends paste or liquid filler for the former, lightly sanded after wiping. Southwick's chart of 11 common woods marks filling as required for ash, butternut, mahogany, red oak, walnut and white oak (Red oak, White oak). Dresdner says gloss finishes and solid colours benefit most, because pores otherwise show and can pack with rubbing compound (Rubbing out a finish). He says fine-pored woods (maple, cherry, poplar, most softwoods) do not need it. Flexner cautions that plain-sawn coarse woods can look "plastic" when filled, and that filler suits mahogany, walnut and quartersawn oak best.

Materials and tools

Filler (oil-based, thinned with mineral spirits or the faster naphtha; or water-based), a plastic spreader or credit card, burlap or cotton rags, and for water-based filler a white nylon abrasive pad. Natural-coloured filler can be tinted with universal colourants; Flexner says not to use dye. Water-based filler is tinted with universal tinting colours (Dresdner), which usually slow drying.

Steps

1. Sand to the final grit and, if staining, stain first (Sanding schedule, Oil-based pigment stain, Gel stain).
2. Flexner's best practice: stain, then a washcoat of about 10% solids (not sanded), then filler.
3. Oil filler. Brush or spray on, let it haze, then wipe across the grain with cotton or burlap, and lightly with the grain at the end. A plastic spreader can scrape it off. Oil filler shrinks, so Flexner uses two applications, the second the next day. Boiled linseed oil slows drying, Japan drier speeds it (Boiled and raw linseed oil).
4. Water-based filler (Dresdner). Scrub into the pores with a white nylon pad, squeegee off the excess with a credit card while wet, and let it dry completely, a few hours depending on humidity. Sand off the dried film with 220 grit self-lubricating paper (any left on the surface shows under the finish), and wipe with a barely damp cloth. Spray a little water on filler that sets up too fast. Cut a credit card to a moulding profile for edges.
5. Remove dust (Dust removal and tack cloths) and begin the finish. Dresdner says water-based filler is compatible with virtually any finish.
6. Or fill with the finish itself. Flexner: 4 to 6 coats of varnish or water-based finish, sanded flat between; shellac and lacquer need more. Dresdner warns the pores can show up again months later as the finish in them shrinks.

Common problems

• Filler too hard to wipe. Wipe with solvent and refill in smaller areas (Flexner).
• Lacquer reopens the pores. Lacquer thinner dissolves filler. Dust the first lacquer coats or put shellac between (Nitrocellulose lacquer, Shellac).
• Pinholes over filled pores. Flexner: bubbles rising from pores in fast-drying finishes, worst when coats are thick and wood is warm. Spray thin dusting coats first (vinyl sealer or shellac for lacquer), or fill the pores and keep the first coats thin (Sanding sealer (and vinyl sealer)).
• Filler left on the surface. Shows as haze under the finish: sand carefully.
• Sources disagree. Dresdner says pore filler does not shrink; Flexner says oil filler does.

Safety and environment

Oil fillers contain solvents and are flammable; rags soaked in oil-based filler can ignite spontaneously (Oily rag safety). Filler is silica-based (Flexner), so wear a respirator when sanding it. Water-based filler has fewer fumes (Dresdner), and Aqua Coat states its filler is non-flammable.

Related

• Sanding sealer (and vinyl sealer), Pre-stain conditioner and washcoat, Sanding schedule
• Oil-based polyurethane, Water-based polyurethane, Spraying with HVLP
• Finishing end grain for the other very absorbent surface