What is Shellac? Shellac is a fast drying transparent finish that is mostly used to seal wood, but can also be used on other surfaces. It can be used as an undercoat or as a top coat. Shellac is a natural substance (made from bugs or something) and has been around forever.
Is it a good undercoating for foam? Here is my experiment:
I used a spray can product called: Zinsser Bulls Eye Shellac and sprayed scrap pieces of foam with 1-4 coats of Shellac. Then I topped the shellac with one coat of metallic spray paint.
You can see my results above. The result wasn’t a smooth finish, but the finish wasn’t bad if you wanted a mated or grainy type finish – like if you were making imitation stone.
Here is a picture of the 4 coat foam after being flexed and you can see where Shellac fell short. Flexing created a lot of tiny little cracks across the entire surface.
Conclusion: While shellac might be a great way to seal that wooden airplane model and I liked that it was non-toxic and “natural”, I don’t think it’s a good choice for foam because of the cracking. I assume cracking would be a problem when used as both a top or bottom coat. And at $7.50 a can, there are a lot of other options which are a better choice.