WOW
JamesI'd have to disagree with Daryl - I guess I just joined a large group...The answer depends on what blistered - the rudder may have been barrier coated or it may not which could indicate the fiberglass blistered with no internal damage or, again, it might not be.If barrier coated, it's a simple matter to see if the glass has also delaminated which is a worst-case scenario. In this event, you'll need to remove the glass and underlying foam to determine if the rudder grid-to-rudder shaft is rusted and repair that, replace the foam, re-glass and paint. It's unlikley you'll need a new rudder.You should easily be able to tell from a close inspection if the blistered marerial is simply the barrier coat or is actually glass. To be safe, you REALLY need to find out why this happened rather than just "bandaid" a repair.