I agree with Greg's preparation system. The abrasion is vital to get good paint adhesion, even if it hurts to loose that lovely gloss gelcoat finish. It is then important to use the correct primer for whatever finishing coat you will use as not all paint systems are compatible.
In the past I have used automotive primers with no problems and recently I have been trying Multi Surface primer from the DIY store, with acrylic topcoat and water based Polyurethane varnish finishing coat - so far so good!

Hi Merchany67
I have just sent you a D/M
Fred

Many different methods to put a decent finish on a hull, from basic Brushing, to rattle can spraying, to Air Brush and regular Spray gun
Most important is to lightly sand the hull with 320 grit paper to remove any residual wax from the molding layup and any oils from fingerprints.
Give the hull a good coat of spray primer, followed with another very light sanding.
Ensure your paint is compatible with the primer, then if brushing, thin out your paint so it brushes on smoothly.
If spraying with rattle can, use steady movements, keep your can 8" to 10" from your hull with light coats until the hull is covered.
Wait two minutes for the paint to tack, then repeat with one more finish coat.
If splitting the finish into two or more colors, mask off the waterline do one colour wait 12 to 24 hours then reverse the masking, then do the alternate colour.
If you get a run in the paint, let it fully cure, then lightly sand the area and go again.
It all takes practice.

As long as the undercoat is a flat sandable primer you'll be in good shape. I like Merchant67's comment about using the same manufacturer for both primer and finish coat... i.e. krylon and rustoleum rattle cans are not always a marriage made in heaven, same with Tamiya and Testors etc.

However, no matter what... I am obsessive about LONG drying times and sometimes even longer gas-out times. I always advise test strips to confirm paint compatibility, but I am pretty obsessive. And no rushing!😉

merchant 67
sorry for insulting you by naming you merchant 76 😊😊😊😊😊
please apologise me

MERCHANT 76
every hull, whatever the material, pvc, or polyester i alawas sand it slightly and then spray a primer overall of the same make as the paint for the topcoat. If that is NOT the case you can start all over again. So read the backside of the primer as well as the topcoat.
Hope this helps and enjoy the building.

What kind of ship is the hull from, are there any pictures?

I am about to embark on a new model and the colour scheme for the hull is blue waterline, light grey then white. Any suggestions on what to use as an undercoat???

Many Thanks in anticipation👍