Hi Jester, the control surfaces of your boat are made from polyurethane resin and should readily accept paint. I would suggest there is contamination of the surface preventing your paint from settling evenly upon it and there are a couple of ways of dealing with it which you may wish to try.
The best method of preparation is to use a pan scourer to flat the surface down, the kind of scourer my wife refers to as 'the green thing'. Next, wipe the surface over with a rag or tissue dipped in cellulose thinners to remove any grease and when dry, lightly dust the surface with aerosol paint and allow it to dry, I use black satin finish spray paint from the car accessory shop and don't bother with a primer as this paint works very well on this type of surface. Once done, give it another slightly heavier dusting and allow that to dry,now finally give it a proper spray over to create a finish.
If this does'nt work, you may have something nasty impregnated in the resin so more drastic action can be taken to rectify the situation: Warm the offending plane with a heat gun but don't get it hot or it may soften. When it is warm to the touch, wipe it with cellulose thinners. Repeat this operation a few times, and try spraying again as per the first instruction. You may find this solves your problem.
I offer this as my own experience and in no way imply your painting techniques are at fault, I hope it is of some use to you.
Best regards.... Chris