Ron,
Have found in painting that what ever surface you start with is really the surface you end up with. Now that I'm painting again I would recommend you get very aggressive with sanding off what is there and get down to the fiberglass.
Tools:
- Belt Sander for rapid removal of larger areas
- Random Orbit Sander for getting a smooth surface
- Triangular "Mouse" sander to get into smaller areas and to do the outside of the toe rail and get a little closer to the corners
- Dremel for grinding out cracks in the fiber glass
The above 4 are to avoid as much hand sanding as possible - you will need to do that between the winch pedestal and toe rail and in other areas. But it is way better to buy a lot of sand paper than wear yourself out with hand work.
Without seeing your deck I would start with belt sander at 60 grit and remove the crazed gel coat. Have used 40 grit and found it to be too aggressive
For random Orbit would use 80 grit to get the surface smoother
After you have removed anything that is cracked on the surface you will have to identify any fiberglass cracks and use the dremel to grind back to un cracked fiberglass. Fill back in using West system epoxy with the pumps and the low density filler. When I started filling back in all of the areas I had ground out I mixed up two pumps worth of the west system and mixed in the low density filler (get the big size of the filler). You need the mixed volume to be peanut butter consistency to prevent sagging - it is amazing how much of the filler dust you need to mix in to achieve this. You won't get it correct right out of the gates but don't worry you can sand off anything that has dribbled when it cures.
You'll need to do multiple passes with the filler as it shrinks somewhat when curing and sags as well so it just takes some extra passes to fill it all in.
Once you've go the surface filled in and sanded back smooth with 80 grit it's time to sand the entire surface with 120 grit with the random orbit sander and "Mouse" sander - you are done with the belt sander as that is only good for rapid removal of what was there to start with.
A good two part barrier coat is a must in my opinion. Durable and you can put either a two part or one part top coat over it. If you use a one part barrier coat you are limited to a one part top coat.
Probably more than you wanted to know. Have found that if I get the surface right the painting goes more quickly. When I rush and move on to painting too soon is when the end result is not a s good or I end up going back to filling in bad surface areas with filler and re-sanding and re barrier coating
Robin