Thanks for all the input so far. Regarding buoyancy, I dont think it a huge issue as the areas being repaired relative to the overall size of the rudder are small. Were talking adding a coupple of pounds to a rudder that weighs hundreds of pounds ( and is still attached). I am starting to lean twd micro balloon mix.
A ? for crissake - you state the foam absorbs water, but then so dous the foam that was from the factory. Am I missing something painfully obvious?