Raising the grain

Overview

Water makes wood fibres swell and stand up, and a dry surface sanded to a fine grit goes rough again the moment a water-based stain or finish touches it. The remedy is to do it deliberately first: wet the surface, let it dry, knock the fuzz off lightly, and then the stain or finish cannot do it again. The USDA Forest Products Laboratory's Wood Handbook adds a second cause for the same symptom: planers crush earlywood cells, and when the wood is wetted the earlywood springs back and pushes the denser latewood out, making the surface bumpy.

When to use

• Before any water-based stain, dye or finish (Water-based pigment stain, Water-soluble dye stain, Alcohol and oil-soluble dye stain, Water-based polyurethane, Waterborne acrylic and urethane-acrylic topcoats).
• On planed wood, and especially wood with thick latewood bands such as southern pine (Southern yellow pine), where the Wood Handbook says grain raise is common.
• Not needed before an oil or solvent finish that does not wet the wood, although the Wood Handbook warns that coatings without water are especially prone to cracking from grain raise, because they never wet the wood before film formation.

Materials and tools

A sponge or lint-free rag and clean water, then 320 to 400 grit paper on a block. Avoid steel wool with water-based finishes: General Finishes warns that particles trapped in the finish will rust.

Steps

1. Sand to your normal final grit (Sanding schedule) and remove the dust (Dust removal and tack cloths).
2. Wipe the whole surface with a damp sponge or rag; it should look wet but not puddle. Wood Handbook: get the wood wet and feel the surface after it dries to see whether it develops bumps or roughness. This is also the diagnosis.
3. Let it dry completely.
4. Sand lightly just to knock the fibres off. Tim Johnson (Popular Woodworking, on pine) dampens and then sands with 400 grit for water-based finishes.
5. Remove the dust and apply the stain or finish. The Wood Handbook says one wetting is typically sufficient.
6. Expect the first coat of any water-based finish to raise the grain a little again, and sand lightly between coats.

Denatured alcohol, as in alcohol-based dye or a shellac sealer, raises the grain a little too (Johnson).

Common problems

• Still rough after the first coat. Normal; sand the cured first coat lightly. See Water-based polyurethane.
• Sanding to #600 beforehand to avoid it. Does not work: Flexner says sanding to a fine grit does not reduce the grain raising caused by water.
• Cracked film on planed surfaces. The Wood Handbook says a film formed before the grain raised is prone to cracking.
• Wetting glue spots. A damp sponge ("dewhiskering") also shows where glue is; see Glue squeeze-out and common finish failures.
• Veneer and thin stock. The sources opened give no guidance on wetting veneer; test on scrap and see Finishing veneers.

Safety and environment

No special hazards: water only. Fine sanding dust still needs a respirator or extraction. Do not use mineral spirits as the check before a water-based finish (Flexner).

Related

• Sanding schedule precedes it; Dust removal and tack cloths follows it
• Wood moisture and acclimation before finishing for wood that is wet to begin with
• Water-based pigment stain, Water-soluble dye stain, Alcohol and oil-soluble dye stain, Waterborne acrylic and urethane-acrylic topcoats
• Spraying with HVLP for sprayed water-based coats