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.
