On my previous Catalina 30, I had two large skin "bubbles" (8" x 4"). Wasn't sure if it was freeze damage. I opened the skin with a vibrating type cutter. Turned out it was not properly filled at the factory. From what I've read, the skin is one problem but the crevice corrosion between the rudder post and the frame is the next big issue. I only had a skin problem. There are a number of you-tube videos on replacing the inside foam.
I did repair the foam void with thickened epoxy and skinned it over with glass and epoxy. Also redid the leading and trailing edges with epoxy and 6" and 3" cloth (eliminated edge fibers by using the 6" and 3" glass ribbon). A useful tip, the epoxy resin was covered with plastic wrap. The epoxy doesn't stick as long the plastic wrap is smooth with no wrinkles. The plastic wrap allows you to move the resin around, keeps it smooth, and keeps the glass and resin in its proper place.
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