5

I recall reading that shellac is supposed to be a "universal" binder which can be used as an intermediate layer between otherwise incompatible layers of finish.

Say, for instance, you are refinishing furniture but you're not sure of the original finish. So the theory goes that you could put on a layer of shellac, and then use whatever alternative finish you want on top of that. (I'm obviously glossing over details here.)

This seems almost too good to be true. Is this a generally accurate attribute of shellac? What are its limitations?

I am aware that some paint primers are shellac-based, which lends some credibility to the idea.

  • Related: this answer: woodworking.stackexchange.com/a/3708/5572 – jdv Apr 18 at 13:40
  • Short answer is yes, but the limitations part is really too broad a query for here (actually for any online venue really) so no hope of something comprehensive for that. – Graphus Apr 18 at 18:29
3

Binder isn't a good word here as in coatings that means the stuff that bonds pigment particles together in paints, e.g. linseed oil in simple oil paints, acrylic/vinyl/styrene suspension in wall paints (US: latex paint, UK: emulsion paints).

When shellac is used in between finishes to help with adhesion or ensure compatibility it's sometimes referred to as a bridge but perhaps most commonly as an intermediate coat or intermediate finish.

Say, for instance, you are refinishing furniture but you're not sure of the original finish. So the theory goes that you could put on a layer of shellac, and then use whatever alternative finish you want on top of that. (I'm obviously glossing over details here.)

Is this a generally accurate attribute of shellac?

Essentially, yes. There's a phrase you'll fairly frequently come across in finishing circles, "shellac sticks to everything and everything sticks to shellac" which basically covers exactly what you're asking about.

While this is a slight exaggeration it does point to shellac's usefulness in this area and it is widely, and justifiably, relied upon for this purpose.

What are its limitations?

As I touch on in a Comment above you can't expect a comprehensive answer to this as it's too broad a subject, possibly requiring a small section or a brief chapter in a book on finishing if the writer was trying to be thorough. But to give you some idea, relating to two fairly common finishing issues faced by woodworkers:

  • shellac can't bond to a non-drying oil, e.g. mineral oil/liquid paraffin/baby oil
  • it won't bond to waxes (basically, only wax sticks to wax)

When I say it can't bond above it should be understood to mean bond well.

As covered at the start shellac is famous for being extremely good at clinging to disparate materials, it'll even bond reasonably well to the slick surface of glass. But in both cases above using shellac is no substitute for thorough cleaning of the existing surface with paint thinner or spirits. However, after this cleaning it may still be advisable to use shellac to help deal with any remaining traces of the two contaminants, and over these the shellac will bond reliably.

0

A couple of provisos:

Stick to dewaxed shellac: If it doesn't say dewaxed, it's not.

Don't used shellac that's been opened longer than a week or so (or if you make your own from flakes). Liquid shellac is hygroscopic (it absorbs water from the air). Old shellac never loses its tackiness (at least here in the Mid-Atlantic.

Shellac can work well as a crawling/ crazing fix for refinishing where chemical/silicon contamination is making finish issues with water based or lacquer finishes.

Stacking disparate finishes is fraught with boobytraps. Especially when you can't identify the old finish(es). Even experienced pros get caught out.

  • "Don't used shellac that's been opened longer than a week or so" Just to check, do you mean left uncapped here? – Graphus May 22 at 6:42
  • Yes; I live almost on the Chesapeake Bay.....hot, high humid summers, cold, damp winters. Even in the middle of winter relative humidity seldom drops below 60%. As such, I've been burned a couple of times trying to use shellac that has been opened even once that is more than two weeks old. I write an opened or made date(in the case of flake shellac) and if its more than two weeks old when I get back to it....I pitch it. – Atcfurnitureservice May 28 at 13:44
  • As many sources mention (and I think I referred to here recently) shellac results are highly variable place to place, user to user. Sometimes hard to pin down why. Where I am humidity rarely dips below 80% and often tops 95 (this is common for most of the British Isles) and shellac products last me well. Current bottle is a commercial product, not shop-made, and had the seal broken early last year. Last time I used it, about a month ago, it was perfectly fine. – Graphus May 28 at 18:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.