Degreasing oily woods

Overview

Some tropical woods carry natural oily resins in the wood itself. According to Bob Flexner in Popular Woodworking, the resin hinders the drying of oils and varnishes, but not of other finishes. The classic examples he gives are teak, wenge, cocobolo and rosewood (Teak has its own entry). The cure is not to scrub out the oil but to clean the face just before finishing, or to block the oil with a barrier coat.

When to use

• Before the first coat of an oil or varnish finish on any naturally oily wood (Boiled and raw linseed oil, Oil-based polyurethane, Hardwax oil).
• When a previous oil or varnish coat on such a wood stayed tacky: Flexner's troubleshooting guide lists oily woods among the causes of a sticky finish.
• You do not need it for finishes the oily resin does not hinder, according to the same source ("other finishes", in his words), though the Wood Handbook warns that finishes may still have trouble sticking to any naturally water-repellent surface.

Materials and tools

Naphtha or acetone (Popular Woodworking recommends either), clean cloths, gloves and a respirator. For the barrier alternative, shellac: Dewaxed shellac or Shellac.

Steps

1. Sand normally (Sanding schedule) and clean off the dust (Dust removal and tack cloths).
2. Water-bead test. The Wood Handbook says that if water beads up on the wood, finishes are likely to have trouble sticking. A few passes with sandpaper are often enough to remove the surface contaminant that causes it.
3. Solvent wipe. Wipe the surface with naphtha or acetone just before the first coat (Popular Woodworking, "Finishing the 5 Types of Woods").
4. Apply the finish after the solvent has evaporated. The wipe is done "just before applying" the first coat.
5. Or seal it. Instead of, or as well as, the solvent wipe, apply a coat of shellac as a barrier; Flexner lists shellac as an effective barrier against the oil in oily woods such as teak and rosewood (Sanding sealer (and vinyl sealer)).
6. Continue with the topcoat. For build coats use wipe-on or brush methods as usual (Wipe-on application).

Common problems

• Sticky or gummy finish that will not harden. The classic symptom. Flexner's cure: wipe with naphtha or acetone before finishing, or apply a shellac sealer coat.
• Water beads on the surface. See the water-bead test; sand lightly, then solvent-wipe.
• Which finish to choose. Flexner's "Finishing Twenty Questions" says to always apply a compatible oil or varnish finish on oily woods; the same source says the oily resin hinders only oils and varnishes. Outdoor use is covered in Outdoor finishing.

Safety and environment

Naphtha and acetone are flammable solvents (check the safety data sheet): ventilate well, keep ignition sources away and wear a respirator. For oily rags and sanding residue see Oily rag safety. Wear a respirator and use dust extraction when sanding any of these woods; see the species pages for each wood's own hazards.

Related

• Finishing resinous and blotchy woods for pine pitch, which shellac also blocks
• Dewaxed shellac, Shellac as barrier coats
• Boiled and raw linseed oil, Oil-based polyurethane, Hardwax oil
• Outdoor finishing, Sanding schedule