I just repaired mine...
I actually worked with the guys from Gougeon Bros. (WEST Marine epoxy...). I believe the rudder is basically a steel frame, with a foam core, covered with polyester gel coat. All I did was drop the rudder, drill about 8-10 holes in the "bubble" or delamination, let the water drain out, let it dry for about 3 months,calmp it down, and filled it with 105 epoxy/206 hardner, fared it out, and repainted it with antifouling. I just put the rudder back on yesterday. Important, if the foam core is damaged (and you can see, once you drill the holes), you may need to re-glass, which is altogether another story. But, with a 28.5, I doubt if that would be the case. My boat is 20 years old. If you have someone do it, your talking anywhere from $700-1000.00 (at least that's what I've found). Lastly, see the archives, and, Foss Foam Rudders.