The easiest filler I found to work with on GRP is "Bad Boy" Plastic Filler. It mixes into a really soft paste that remains slightly flexible when cured. Sands nicely too and blends in without problem.
I don't know if its an approved method but I use a 12"or 18" flexible plastic ruler to blend the filler to roughly the right shape of the panel. It gets the contours very close so minimal sanding is required.
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