Don't have any experience with fiberglass, but the one spot that I sanded to bare metal and primed, is the one spot where my herculiner has peeled. I used a self etching primer, and let it dry 24 hours.
That herculiner would stick to anything. Just herculined my jeep 2 weeks ago and that stuff is tuff. Did the gas tank cover also. Also did a 4x4 section on my garage floor. It stuck to some bondo i had on holes in the tub, no problem. I brushed the hole thing with no problem, New brush each coat. Glad to have no carpet and can wash out if i need to. Have fun!!!
I herculined the entire exterior of the XJ that's in my signature pic. The rear hatch on XJ's are I believe some sort of fiberglass..it's not metal or plastic, and it stuck to that and Everything else it touched without any problems.
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