In the old days you could have slapped some nitromors paint stripper on it and let it pickle for a while, or stick the B&D sander over it,But now what with "Elf&Safety you have to think about your health,and quite right too.
If the old paint is firm then just give a light sanding down,preferably outside,and do not sand the fibreglass to much as the fibres are horrid, and WEAR a MASK!
Good luck,
Len