Hi
You need to grind out the crack at a shallow angle with an abrasive disc (P80? - been a while), then and then add fibre glass matt or tissue and resin, then fill and paint. There is no quick fix.
The best book for grp car repairs was by Miles Wilkins, 'How to Restore Fibreglass Bodywork' - unfortunately long out of print. But you can now buy a reprint from a Lotus Books in Germany. It comes combined with Wilkins' book on car painting. See:
http://www.lotus-books.com/additions.html You can pay in £. Order form:
http://www.lotus-books.com/order.html Highly recommended, particularly as very little has been published on grp car repair. Recent articles in Practical Classics have been wrong, it won't work long-term to just do a narrow grind out of a gel crack and put resin/'gelcoat' and/or filler and then paint. This contradicts info they published years ago from Miles! Got to do wider grinded area and then mat/tissue and resin, fill and paint.
If anyone needs to buy fibreglass supplies in Scotland, I've found Allscot very helpful. Trade prices, DIY quantities, and open late some evenings as well as Sat morning. And no, I don't work for them, or get a discount!
Web:
http://www.allscotltd.co.uk They're in Glasgow, but do mail order
Cheers
Mike