Finbox repair- advice welcome

[RadarOreilly]

I bought this used displacement SUP for my wife - the finbox had this delamination damage on the one side - the board probably got dropped, etc. The finbox itself is stable, straight etc. The board is watertight. I found the board otherwise sound and a great $ deal, and took this on as a project.

I'm ok at repairing simple dings, etc but this repair has been trickier than I thought. Any advice welcome. I fixed by grinding out all around the delamination, cleaned it out to solid material, then laid in 2 layers of glass and resin over the finbox and built up back to level using a DingAll epoxy resin repair kit.

Everything has gone pretty well but 2 issues now:

1. I can't seem to get a clean edge around the finbox - every layer of resin I add won't give me that clean square edge I'm looking for. Should I try an epoxy putty to get that square edge? Or keep going by somehow taping off the finbox and building up more layers of resin? I can't figure it out. (see pics from before and after below)

2. After coloring with (white) acrylic paint, I tried to put on a final "gloss coat" or whatever with the Dingall epoxy, and these weird sort of cat paw prints appeared - even though I thought I sanded completely, the last coat didn't want to bind to the Acrylic paint I guess. (see last photo below. )

Any helpful advice welcome

[waikikikichan]

The finbox itself is stable, straight etc. The board is watertight. I found the board otherwise sound

I know you wrote it, but how are certain you know it ? That the box was watertight ?

[RadarOreilly]

Well, I guess by paddling it around and not hearing any water retained in the (hollow) hull. Also by visual inspection. I'm curious, how would you know the board was *not* watertight?
I understand that with a foam board this might not be as clear-cut. With this board it seems more clear, but I'm here to learn.

[waikikikichan]

I tried finding a construction cutaway of a Boardworks board, but couldn't. But most of the Molded-Epoxy board manufacturers use similar construction techniques. They are not hollow on the inside.

Usually you can find a build up of salt crystals around the area that is taking in water. A lot of times "cracks" on a molded-epoxy board is just the top cosmetic layer ( like i think was in your case ). Boardworks had horrendous paint jobs. I'll post later a photo of my Aipa big boy sting made by Boardsworks, where the top layer of paint has fallen off and you can see the fiberglass layer that is solidly intact.

I asked how you knew, since Boardworks were infamous for their "bandit" Futures sidebite boxes. The back pins would fall out and the dealers had to tell customers to glue in the fins. Also the boxes would leak as they found out during the production process only one side was glued in.