What is shellac, and when should I use it?

Shellac is secreted by an insect, harvested, refined into flakes and dissolved in alcohol. It has been in use for centuries and nothing has replaced it, because it does something no modern product quite matches: it bonds to surfaces that reject other coatings, it blocks contaminants that migrate through other coatings, and it is compatible underneath almost everything. Knowing what it is turns three separate decorating problems into one answer.

Safety

Step by step

1. Use it as knotting solution on bare softwood.Two thin coats on each knot and the wood immediately around it, applied to bare timber before priming. Shellac is impermeable to the resin that bleeds from knots, which is why it has been the standard answer for a century. Knotting solution sold in decorators' merchants is exactly this.
2. Use it as a stain block.A pigmented shellac-based primer is the most reliable barrier against water stains, nicotine, soot, felt tip, crayon and timber tannin — all of which are water-soluble or resin-soluble and pass through ordinary water-based paint. It dries in minutes, so a stained ceiling can be blocked and painted the same morning.
3. Use it as a barrier between incompatible finishes.Water-based varnish over an oil stain, a modern lacquer over an unknown old finish, or a finish over a surface you are not sure about: a coat of dewaxed shellac between them isolates the two. This is the traditional and still the standard solution to a compatibility problem.
4. Use it as a sanding sealer.Thinned shellac soaks into bare timber, stiffens the raised fibres so they shear off cleanly when sanded, and evens out absorbency before a stain or a finish. On blotch-prone woods like pine and cherry it works as a pre-stain conditioner.
5. Buy dewaxed shellac if anything is going over it.Natural shellac contains wax, and some finishes — notably polyurethane — adhere poorly over it. Dewaxed shellac is universally overcoatable and is what you want for barrier and sealer work. Ordinary orange shellac and standard knotting are fine directly under paint.
6. Know its limits as a finish.As a finish in its own right, shellac gives a beautiful warm depth and is easily repaired — which is why French-polished antiques are finished with it — but water rings it, alcohol dissolves it and heat softens it. It belongs on furniture that is admired rather than used.
7. Handle the practicalities.It dissolves in methylated spirit and nothing else, so brushes must be cleaned in meths immediately or thrown away. Made-up shellac has a limited shelf life and stops drying hard when it is old — if a coat stays soft, suspect the age of the tin. Flakes dissolved fresh keep indefinitely as flakes.

Tips

• Thin coats, applied quickly, and do not go back over an area — shellac sets so fast that a second pass drags the first.
• If a shellac coat stays tacky, the product is almost certainly out of date. It is the single most common shellac problem and it is not fixable by waiting.
• A shellac-based stain-blocking primer in an aerosol is the fastest way to deal with a ceiling water stain, and it saves the brush.

Common mistakes

• Using waxy shellac under polyurethane — The wax interferes with adhesion. Use dewaxed shellac wherever another finish is going over it.
• Using shellac as a finish on a table or a worktop — Water marks it, heat softens it and alcohol dissolves it. It is a barrier and a sealer, and a finish only on pieces that are not used hard.
• Cleaning shellac brushes in white spirit or water — Neither dissolves it. Methylated spirit is the only solvent, and the brush has to be cleaned immediately.

Questions people ask