Steve Firth wrote:

The usual gap filling ahdesive used on GRP is epoxy filled with glass
microballoons.

Most GRP is styrene, not epoxy. If you are filling styrene based GRP,
you really need to use styrene based filler - otherwise adhesion isn't
great.

The majority of car body fillers are styrene based, and so work well on GRP.


--
Grunff