Spider cracks in deck

[SkunkBoat]

The cracks are usually just the gel coat. You have to widen the cracks and taper them out, then use a filler/resin. I would probably use epoxy and lightweight filler in that case.
You could use pre-made polyester filler(Bondo).
I have used both in different places with success.

If you are using a textured paint, you could maybe use one thin layer of glass cloth. You should use primer before paint.