Spray foam for filling big tear outs in EPS

I tore a 10" box out of the bottom of my board and it took a lot of foam out with it. I lost the fin, box, and a shit load of EPS that tore out in the process. So I bought a can of DAP KWIKI spray foam (polyurethene) and sprayed the hole. It is quite a big hole so I had to spray a lot of foam out( 2’’ deep x 6" x 12") I’ll give it a day to set up nice then sand it down and install a box and glass it. I think that it will work well. The color of it is a bit yellow so Ill have to spray a section to cover it. Has anyone ever tried what I am trying, if so how did it work out?

It works,

There’s also styro in a can or mix too…or just get some styro from packing and cut and glue to fit.

I’m so happy to hear this works. I was installing some X2 plugs for a quad/canard setup and there was a small cavity I didn’t notice between one canard plug and the nearest trailing fin plug, and some of the epoxy seeped through so instead of two separate holes with a small amount of epoxy it became one hole with a bit more than I would like in a single pour. The one plug started smoking and I had to rip it out, but I unfortunately not before it made a hole between the plugs in the foam.
None of the other plugs smoked out so I know I didn’t pour it too deep. I do remember now there’s many reasons I stopped using these and changed over to fusions.

I panicked until I saw this

Rout the hole to get the even shape and fill it with a foam plug from your offcuts.  Gorilla Glue or epoxy it in, sand it flush with the blank and glass.  You get the same texture and flex as the rest of the blank because it’s the same foam.   No danger of exotherm or adhesion problems.  You’re using scraps, so that’s arguably a better use of your existing resources and a reduction of your waste.  

For finplugs and box installs I use a plug from my stash of HD PU foam.  

True, typically that is what I would do - but this is a custom wakesurf board for my brother and it has a colored petina lam job that I don’t want to destroy by cutting out more than I need to - I’m not worried so much about flex/strength for two reasons: One, given these are x2 plugs they’re supposed to be bonded to the deck-side fiberglass anyway, so in theory it should be just as strong as your typical install if done properly… second is that this is a wakesurf board, so the forces exerted on the fins will be exponentially less than those on typical ocean surf. 

In the future I’ll stick with glass-over boxes like fusion or future… I knew there was a reason I liked them better - trying to save myself a few bucks really bit me in the ass

It’s best to increase the density of the spray foam for strength. 

Hace a piece of wax paper and brick or weight at the ready and spray a bunch of foam in the hole. 

Quickly cover the hole with the wax paper and weight so the expanding foam gets trapped and compacts itself to a higher density

That’s good idea actually, I was wondering about trying to manipulate the density like that. Think it will hinder cure time being sealed off with wax paper?