You suggest not using an ablative paint, which means you are looking for an antifouling paint, right? Ablative (multi-season) paints are perfect for trailerable boats, if you are looking for something to keep your rudder from fouling. Ablative paints are designed to leach off of your rudder or boat bottom as you use the boat. So, like a bar of soap, it gets smaller with each use, but continues to clean until it is gone. That's the way ablative paints work. That way you don't get build up over time and when you see bare fiberglass, you know it is time to repaint. Secondly, most single season paints have a cuprous oxide biocide in them which work great as long as the boat sits in the water. But once you take it out and the surface dries, these copper paints are rendered less effective, because of a chemical change, while with an ablative paint, you can remove your boat from the water and let it dry as many times as you want and when you relaunch, you can expect good antifouling protection. Single season paints work differently. The biocide leaches away from the paint and leaves the paint behind, which eventually leads up to paint build up over time as you continue to recoat. So, you can look at an old paint application, which might look good, but cannot assume there is any biocide in the paint to protect your hull. And, if there is, you have to abraze the surface to open up the pores to release what biocide is left, which is a job and a half. With ablative paint, as long as you see paint, you know it's working for you and it doesn't pile up on the hull because it sloughs off until it is gone.
Now, if you are looking for a different method of finishing your rudder without use an antifouling paint, then an epoxy paint would be a good choice. Especially a two part epoxy. But, remember while epoxy is pretty waterproof, it doesn't hold up to sunlight well and hence will fade and oxidize more quickly than a topside paint. Most professionals use a polyurethane paint like Awlgrip, which is a linear polyurethane top coat. But these are never used under the water line where any lengthy exposure exists, because they will blister. One part urethane paints like Interlux' Brightside will also blister under the waterline. They are fine above the waterline, where they get wet, but not immersed for any length of time.
If the above does not answer your question, then LMK.
