How to make repairs to a gelcoat hull

What is the best way to repair a fibreglass hull, which is approx 25 years old? Once I have fixed the problems below, the hull will be painted with two-pack polyurethane.

1. Hairline cracks in the gelcoat ranging from 20 to 500 mm in length.
2. Small half circular cracks, approximately 4 mm round and slightly raised. They are quite hard with no moisture underneath.
I was going to grind the hairline cracks out slightly to give filler something to hold onto and then use either poly or epoxy filler, but which one given that the hull will be painted.
The semi circular cracks well I am not sure how to solve this one, could the hull be thoroughly cleaned to remove any old polish/wax and dirt with sugar soap. Then once dried, painted over with a thin poly or epoxy resin to seal the surface and finally sanded back flush with the hull????
Am I on the right track or barking up the wrong tree?

Regards
Bruce