I had the same problem with my ribbed factory boards on my 1950 3600. For the major holes I pulled the boards off and used fiberglass from the bottom side. I bought the actual resin and not the jelly, and I used the matting that looks like cloth. Then I used "dyna-hair" filler on the top side rather than traditional plastic filler to make the final prep. It was a little harder to sand, but it worked like a charm. I have had two years now with no problems.
