Here's my .02. I have done alot of work with paint and fillers from back in my autobody days and also trial and error on my Kona. First I was told to use mar-glass (bondo with kitty hair). It is stronger then standard bondo but has no flex in it what so ever so as your boat is hitting wakes and such it can stress cracks, so I filled my bump with mar and did some light sanding and sat on the side rail after a while I heard it split what I found is if you sit on the inner edge it pulls the seam apart causing the crack. So I cut a few triangles to releave some of the load and glassed them about 10in apart to the side and top and filled the rail and did some light sanding now I can stomp, sit, make love on the side with out cracking hope this helps Daniel