Thanks onetenor

If the deck is also GRP the best bet is to plank it with fine veneer strips then varnish that. if it needs filler between the planks use a black one. Carbon black or similar added to the filler .Brown would also look smart.Artists powder paint could also be used and be cleaner to handle.Lite filler from B&Q is very good .Light and easy sanding and I believe slightly flexible so should not crack out.

Thanks....I was hoping to sand the deck back to a state where I could varnish rather than paint....quite happy to re-paint the hull etc

Sonar is right. Do not try to sand it ALL off as you risk damaging the gel coat of GRP Almost impossible to rectify .BE WARNED!
John

Wet and dry sandpaper and lots of washing up liquid go down through the grades of paper til 320 grit

The worst thing you can do it try to send it all back

Better to send and get a good base to start again with spray paint