I used Interlux Perfection on my daughter's Santana 22. The original paint was cracked, pitted, and needless to say, generally tired. I pressure washed the boat, filled all the unused holes (thickened epoxy) and obvious defects, then gave it a thorough sanding - not down to through the gelcoat, but sufficient to get an even, clean surface and "key" for the new paint.
(I should add here that I have seen it written that you should sand off all the old gelcoat, otherwise the primer would react with it and they would peel off; this did not happen to me, but more research may be needed; I did speak with Interlux, who said the primekote would be fine, and they also said it is great for filling small cracks).
I then gave it two coats of the Interlux Primekote. After sanding (one tends to do a lot of that), all cracks were filled, and the surface was smooth and ready for final fairing and the topcoat.
2 coats of perfection, using the 'roll and tip" method. Perfection is expensive, and as a 2-part paint needs to be applied quickly on a cooler day. However, it went on very smoothly, and ended up looking great. It has been around 5 years; the boat has a new owner, and upkeep has not been so good...but the paint job still looks like new.
Hope this helps.