What board is it?

Is the epoxy mix getting hot? If epoxy gets hot it will melt EPS.

I once developed a method for repairing boards with smashed noses. You open the nose, get a screwdriver and dig out all the soft foam, and a bit more for extra measure. Stand the board upright, and then pour thickened epoxy in the hole. This turns the nose into a brick. Then sand it back and paint over it. (these were company boards)

Sometimes the mix would get super hot so have a bucket ready to dunk the nose in before the board catches fire..

Are you sure you're using epoxy?????
Unless as Barn says straight epoxy is getting too hot to touch it won't melt the foam. If you've added a thinner to it that will probably do it as well.
Polyester resin will also do what you have described, don't get it anywhere near styrofoam

I'd sand the expandafoam back to just below the existing surface feather the edge back at least 2cm, then use fibreglass cloth, (not bits of fibreglass) and pure epoxy.

Darkplague said...

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I used Fibreglass resin (Diggers brand) from Bunnings
Just assumed it was epoxy based resin, maybe I better check the label, oops

Yep, that's polyester I'm afraid.
A quick way to tell is the mix ratio, polyester is about 1% hardener to resin, while epoxy typically varies between 20% and 50%

And normally the only epoxy you'll get from places like Bunnings is araldite

Melting the styro core even with epoxy is also common if the epoxy is too liquid and runs into the foam.
Very common on production boards which get built in a hurry and in a mold.
Plenty of epoxy runs in and melts styro everywhere.
I've sawn up quite a few dead prod boards and in all the spots where there was an external ding or crack, there was a void in the styro core where melting had taken place.
One board had a huge void under the mast track and the track had dropped in letting in water.Also very common with footstrap inserts.
The cure is to mix the epoxy with some filler to get it to butter form and then smear this over the core, wait until it gets a bit tacky and then put your fabric and insert into the hole.This 'skin' tends to keep the heat away while the cure takes place.
And when you're laminating, wet the fabric out on a poly lined table then roll the fabric onto a poster tube to transfer it to the blank. This leaves most of the excess epoxy on the table rather on the job.
Keep the excess heat away and you get good solid boards.

I had a fine crack in the nose of my board and used some araldite to cover/fill it..that should be ok shouldn't it?

Araldite- good. The polyurethane expandafoam stuff seems to have not much strength, even when set unbder pressure. Contact adhesive is a no-no.

What other stuff DOESNT make great holes in EPS?

Araldite works well for temporary repairs but it won't last for a long time.

I've been using the west system epoxy and some of the repairs with it are over 5 years old and still good