fiberglass repairs

The roof on my 110 is made out of fiberglass and has a number of cracks in the gel-coat. I am about to start tidying it up so i can paint it. I only have limited knowledge with fiberglass.
What is required to be done to prevent the paint from cracking. Do you have to sand the cracks down to the fiberglass or is a thick coat of primer enough to seal them. I don't wont to have to go back and repair the roof in a few years time.
Any advice?

sand it back enough to remove cracked pait or gel coat ,skim with decent filler you can get waterproof for boats ,then prime otherwise crack will just reappear

thanks James i was afraid of having to sand them out. but has to be done.

worth getting a good da sander and decent filler .dont be afraid ive just done me little boat,even flexing in the sea hasnt reintroduced any new surface cracks ,a real good quality top coat polyurathene or 2 pack is the key to keeping water out and lifting paint later

Yeah it will be painted in 2 pack truck paint with good quality primer. I was reading you can use a dremel or die grinder to open up the cracks in the gel coat. Have you tried this? Most the minor cracks are a star like pattern on the top surfaces. And large cracks around the roof ribs. If only it was a alloy roof

yes were you have large cracks that go through fibre glass i cleaned thorughly with sanding put small patch on underneath then painted resin on top in crack then filled ,where and cracking goes to fibre glass even if not completely through i resined then sanded filled as cracking in gel coat that had been done before in its life had failed you need to be careful though as fibre glass can soon disappear and leave you with more work a dremel i should think ideal, i didnt like fibre glass before but after working with it changed me mind in that you can add and sand away easily a bit like steel in away,

Interesting reading. The only places where it has cracked right through is up around the inside where the roof meets the windscreen. I guess this is the reason for one of the water leaks into the cab. I am also wondering if i should modify the roof gutter to come around to the windscreen. Its seems to overflow down the front side windows which makes getting in with out a wet leg almost impossible. I don't remember having this issue when i had my series 3. Is this a common problem with 110 or defenders?

series 3 guttering goes right round the front defs just lead slightly forward because windscreen enchroaches into roof