Well to tell you the truth, Whoever owned my boat before had painted the rudder with bottom anti-fouling
paint. So I sanded my rudder completely, you are doing fiberglass work so I would sand it down real good using an orbital sander and maybe 80 grit for the thick tough spots. Wipe it down afterwards with acetone or something before you do the fiberglass work and try to get the dust blown out as well *(Wear eye protection and a dust mask!)
As for the painting part.... Well like I say a coat of straight
epoxy is your best water proof bet. I had some issues with the
epoxy actually beading on me, not sure if the problem was from not sanding enough or not thinning the
epoxy enough. At any rate there are several tests out there showing that a coat or two of plain
epoxy will get you a whole lot more water protection than anything.
Once the "Barrier" coat is on and sanded with fine sandpaper you are ready to
paint. Check around to see what
paint is compatible with your epoxy, I have actually used enamel porch
paint in the past! It held up amazingly well and looked pretty good too! There are all kinds of linear poly paints, epoxy paint you name it lots of choices, depends on your use, your budget etc.
One word of caution though, I have heard of problems with rudders outgassing and splitting when painted a dark color! The extra heat supposedly heats up the chemicals, or the water still trapped inside which expands and splits the rudder. Not sure if I believe it but worth mentioning I guess. every time I go to the marina and see a split rudder It is always white.
But as far as sanding the paint off, yes! You are going to have to resign yourself to reworking your whole rudder if its a large split. I don't know exactly what all is wrong with yours, it is split & somebody drilled holes in it or whatever, usually it is tough to just do spot work *(Gel coat issues). If you just have a small crack and a couple of tiny holes for draining it....Yes you can just do a small
repair. But like I say with mine I split it as much as I could and really went after it. Besides I had to get that stupid paint off the rudder! And I have yet to see a split rudder that I would consider a minor cosmetic
repair, every one I have seen is a major structural issue.
Examine it carefully and you may see a few cracks near the hole for the tiller where extreme stress has been really flexing it. On the bottom corners where it has been grounded and rubbed a hole. Now is the time to do it right once and for all.
Don't worry by the time you are finished with this rudder, sailing season should just be starting up.....09' anyway!
Good luck let me know if there is anything else I can confuse you about!
Luckytexan