Dewaxed shellac

Overview

Dewaxed shellac is ordinary Shellac with the 3 to 5 percent natural wax removed (Wikipedia). It is used where a topcoat must adhere, and it is the usual choice for a barrier coat against bleed, knots and blotching. The film is thin and fast, and it is rarely the final surface.

Pros

• Topcoats can stick to it where they might not stick to waxy shellac; Wikipedia says dewaxed shellac is used where a topcoat must adhere.
• Seals in knots and resin; the USDA Wood Handbook names shellac for sealing knots.
• Dries in minutes, so a barrier coat adds little time to a job.
• Same alcohol clean-up and easy repair as ordinary shellac.

Cons

• Same weaknesses as shellac: only fair water resistance, poor solvent resistance (Wikipedia).
• Not a protective finish on its own for a tabletop or bar.
• Liquid shellac stored for about a year goes off (Wikipedia); a drop on glass that stays tacky means the can is finished.
• Canned products are not always labelled dewaxed. The Zinsser page we found carried no usable technical data, so read the label.

Suitability

Best as an undercoat on resinous softwoods such as Knotty pine, Eastern white pine and Southern yellow pine, where it blocks knot and resin bleed (Wood Handbook for knots), and as a seal over oil or wax-contaminated surfaces under another film. As a wash coat it can reduce stain blotching (Pre-stain conditioner and washcoat). Skip it where the final film must resist alcohol or water; put a varnish or polyurethane over it (Oil-based polyurethane, Water-based polyurethane).

Coats and application

Sand to 180 to 220 grit, remove dust, then apply one thin coat (choose the cut your topcoat's data sheet allows; one pound of flakes per gallon of alcohol is a 1 lb cut, about 120 g/L per Wikipedia). Let it dry, then scuff lightly with fine paper and apply the topcoat. Brush quickly or pad it on (Wipe-on application); do not overwork the film. Check the topcoat's data sheet for the maximum shellac cut it accepts.

Compatibility

• This is its job: a compatible base under Oil-based polyurethane, Water-based polyurethane, Alkyd varnish and Nitrocellulose lacquer. Follow the topcoat's own data sheet for which shellac and cut it accepts.
• Do not use waxy shellac as the barrier unless the topcoat's maker allows it.
• Alcohol-based dye stains under shellac are fine if fully dry (Alcohol and oil-soluble dye stain); see Sanding sealer (and vinyl sealer) for the general sealer role.

Care and repair

Care is the topcoat's. If shellac is the surface coat, wipe with a damp cloth and avoid alcohol. It re-melts with alcohol, so repairs are a touch-up coat; stripping is denatured alcohol.

Safety and environment

Alcohol vapour is flammable: ventilate and keep ignition sources away. The federal VOC limit for clear shellac is 730 g/L (40 CFR 59 Subpart D), a national figure. Shellac is a named natural resin in 21 CFR 175.300 for food-contact articles, but a can of woodworking shellac may include denaturants, and finished furniture is not certified by that regulation. Use the maker's own statement, and see Food-safe finishes.

Related

• Shellac: the general entry, including pound cuts and French polish
• Sanding sealer (and vinyl sealer): other sealers for under a topcoat
• Grain filler (pore filler): pore filling that comes before or after the seal
• Finishing end grain: end grain often wants extra sealing
• Oil-based polyurethane and Water-based polyurethane: common topcoats over a seal coat