Les,
A few years ago I part-owned a GRP sailplane (Libelle 15m) and we needed to do the self same job - GRP quickly degrades at altitudes with lots of UV light.
Question: How bad is the GelCoat?
Answer: Just a bit dull - try some T Cut and a lot of rubbing followed by a special GRP wax dressing and more rubbing.
Answer: Very dull and matt - chalky in places - if you rub with your finger a layer of white powder comes off.
Get some 600 and 2000 grade wet and dry paper, and a bucket of water with one or two drops of detergent.
Starting with the coarse 600 grit paper (well compared to 2000 grit it's coarse), wet the paper and the van roof and lightly rub to remove the chalky area(s). Just a few strokes hsould be all that's needed. Keep the roof and paper wet at all times. When happy, change to the fine 2000 grit paper and repeat.
Don't rub all the way through the gel coat - depending on the maker this should be around 1mm thick before you get down to the bulk of the glass mat / clear resin layers.
Then do the T cut and wax polish thingy.
Answer: Really bad - lots of chalkiness, areas of gelcoat missing. A repair with a top coat repair paint is the answer.
Do the 600 grit thingy again, but don't bother with the 2000 grit and definitely no T cut or wax. Then carefully clean the roof and remove all dust. Then apply the gelcoat paint with a spray gun or fine roller - gun is better - you may want to get this done professionally.
Robert
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