On Fri, 26 Feb 2016 17:55:13 -0800 (PST), wrote:

I find that paint remover attacks quite a few various "things". Porcelain should be untouched by it, but I don't know about fiberglass. Whatever you try, it may take a few hours to even start to soften it. Single-edge razor in a holder is what I would try first, then the paint remover on whatever is left.

Back months ago when I had some deposits on my bathroom sink and after
discussing it here decided to use wood popsicle sticks to push the
stuff off... it worked well but months later, there was a chip in the
porcelain where I'd been working. A much smaller area than what I
cleaned, but I think I cause it.