Wood bleaching

Overview

Wood bleach removes or lightens natural colour or stains. Wood Magazine says only a two-part sodium hydroxide plus hydrogen peroxide bleach removes natural colour, while chlorine removes dye and oxalic acid removes water and rust stains. Popular Woodworking lists four types. Fine Woodworking (Jewitt) lists uses: match old wood to new, even out different woods, and prepare a blond or pickled finish.

Pros

• Evens colour between boards and removes stains.
• Two-part bleach removes natural colour, giving a pale base for Liming wax and cerusing or blond finishes.
• Oxalic acid removes water and rust marks.

Cons

• Hazardous chemicals.
• Fine Woodworking says lye can darken first and can blind and burn.
• More than 2 coats raise grain and can give a green cast (Jewitt).
• Chlorine leaves salt residue and removes dye but not pigment (Popular Woodworking).
• Maker caution: residue can affect water-based products (Liberon). Results are not guaranteed.

Suitability

Honduran, Philippine and African mahogany bleach well; Cuban darkens (Fine Woodworking). See Honduran mahogany. Blue stain on maple: Popular Woodworking says 35% peroxide.

Coats and application

1. Mix two-part bleach 1:1 in glass or plastic. Fine Woodworking (Robbins) used Daly's 1:1 with 10 to 15% extra part B; the mix stays active about 30 minutes; 3 coats for white; sand to 220.
2. Wood Magazine: rinse, then white vinegar, then water; sand 320 to 400. Jewitt: Klean-Strip 1:1 weakens after 3 to 4 hours; neutralize with vinegar and water after 2+ hours; dry 24 hours; sand 180 to 220.
3. Popular Woodworking: chlorine at about 1 qt per gallon kills mildew; A/B lye bleach darkens first, dry overnight, wash; use long neoprene gloves; oxalic acid 1 oz per pint of warm water.
4. Liberon: rinse 2 to 3 times; some stains resist.

Compatibility

Neutralize and rinse thoroughly before finishing. Raise the grain and sand (Raising the grain). Follow with colour or clear (Water-soluble dye stain).

Care and repair

Re-bleach if colour returns, but the wood weakens with repeat treatments. Do not bleach after finishing.

Safety and environment

NIOSH: sodium hydroxide ceiling REL 2 mg/m3, PEL 2 mg/m3, IDLH 10 mg/m3, causes burns. Oxalic acid: REL TWA 1 mg/m3, ST 2, IDLH 500 mg/m3, kidney damage. Hydrogen peroxide: TWA 1 ppm, IDLH 75 ppm, powerful oxidizer, incompatible with iron, copper and brass. Wear rubber gloves, glasses and a dust mask (Jewitt). Never mix bleach with ammonia.

Related

• Ammonia fuming
• Lye and chemical staining
• Liming wax and cerusing