Shellac

Overview

Shellac is a resin secreted by the lac bug, sold as dry flakes and dissolved in alcohol to make a liquid finish. It dries by evaporation to a thin, clear to amber film, and it was the dominant western wood finish until nitrocellulose lacquer largely replaced it in the 1920s and 1930s (Wikipedia). Woodworkers still choose it for speed, for how easily it repairs, and for its job as a barrier coat under other finishes.

Pros

• Dries in minutes and recoats quickly; the same solvent re-melts the previous coat so coats burn into one another.
• Completely reversible with alcohol, so scratches and failures can be wiped or padded out (Wikipedia's finish comparison lists it as "completely reversible" with excellent rubbing qualities).
• Seals knots, resin and many stains; a standard barrier against bleed and blotching. The USDA Wood Handbook names shellac for sealing knots.
• Almost any other finish can go over it once it is dewaxed, and it sticks to nearly anything.
• Natural resin with a long record of use as a food glaze and pill coating.

Cons

• Only fair against water and poor against solvents, including the alcohol in drinks and some cleaners (Wikipedia). Not a good bar top or tabletop finish.
• Moderate wear resistance; it is a thin film.
• The solvent flashes so fast that brushing takes practice; Wikipedia says it clogs spray equipment, and recommends pads or natural-hair brushes.
• Liquid shellac has a limited shelf life, about one year per Wikipedia. Old stock that stays tacky on a glass drop test should be discarded.
• Ordinary shellac carries 3 to 5 percent wax (Wikipedia), so a waxy shellac is a poor base for an adhesion-critical topcoat. Use Dewaxed shellac there.

Suitability

Good for interior furniture, trim, turnings, and as a sealer or knot-blocker on resinous softwoods such as Knotty pine and Eastern white pine. Wikipedia lists blotch control as one of its uses, so a thin coat is a common first step on uneven-absorbing hardwoods such as Hard maple; compare Pre-stain conditioner and washcoat. On open-pored woods such as Red oak the pores will still show unless filled first (Grain filler (pore filler)).

Not suited to dining tables, bar tops, bathrooms, or anything that sees spirits, hot cups, or standing water. Wikipedia rates it UV-resistant, but the amber colour of the resin itself will warm pale woods.

Coats and application

Prepare by sanding to 180 to 220 grit (Sanding schedule). Mix flakes by pound cut: one pound of flakes in a gallon of denatured alcohol is a 1 lb cut, about 120 g/L. Premixed canned shellac is mostly a 3 lb cut (Wikipedia), so thin it with alcohol to a 1 to 2 lb cut for brushing or padding.

Example schedule: wash coat of a 1 lb cut, sand lightly with 320, then two or three coats of a 1.5 to 2 lb cut applied with a pad (Wipe-on application) or a good brush, with a light 320 scuff between. Move fast and do not overwork a coat that has started to set. A mirror finish is built by French polish and padding, and the surface can be Rubbing out a finish after it hardens. Check the can for recoat and cure times; no figure is quoted here.

Compatibility

• It sticks to nearly anything and most other finishes will go over it, but wax in the shellac can interfere with topcoat adhesion. Use Dewaxed shellac under varnish, polyurethane, lacquer or Water-based polyurethane. See Sanding sealer (and vinyl sealer) for the sealer role.
• Wikipedia notes that virtually any other finish can be used over shellac; it is used under Nitrocellulose lacquer and similar finishes as a sealer. Check the topcoat maker's data sheet for the shellac they accept.
• Over stains: let dye and oil stains dry completely first (Alcohol and oil-soluble dye stain, Oil-based pigment stain).
• Do not put it where alcohol-based cleaners or drinks are regular guests.

Care and repair

Wipe with a damp cloth; avoid alcohol-based polishes. Light wear can be revived with a fresh thin coat or a pad of alcohol-thinned shellac, because new coats melt into old ones. Stripping is easy with denatured alcohol.

Safety and environment

The solvent is flammable alcohol: no open flame, ventilate, and keep it away from sparks. The federal VOC limit for clear shellac is 730 g/L (opaque 550) under 40 CFR 59 Subpart D, so expect a high solvent content; this is a national limit, and states may differ. Shellac is a named natural resin in the FDA's food-contact coating regulation (21 CFR 175.300) and is used as a food glaze, but a woodworking-grade can may contain denaturants and additives, and a finished piece is not certified "food safe" by that fact. Look for the maker's own statement on the label. Treat alcohol-soaked rags as flammable waste; see Oily rag safety for rags from oil-based products.

Related

• Dewaxed shellac: the variant to use under other finishes
• French polish and padding: the high-gloss padding technique
• Sanding sealer (and vinyl sealer): shellac in its sealer role
• Pre-stain conditioner and washcoat: a wash coat of shellac can do the same job
• Nitrocellulose lacquer: the finish that replaced it commercially
• Food-safe finishes: how to read food-contact claims