One thing that I have discovered works fairly well for strengthening foam is to use cheesecloth. You take pieces of it, press them to the foam surface, and apply a coat of paint to it. The paint will cause the cheesecloth to stick to the surface and harden a bit. You then put a thin layer of Bondo over the top of that and sand it down until it is smooth.
That should get you some nice looking results. Not sure if I would use resin with foam.
I should add though that it also helps if you have something to strengthen the length of the weapon with. Cheesecloth and Bondo will certain protect the foam from a lot of surface damage, but will not necessarily protect the weapon from getting snapped in half due to forces in unfortunate positions of leverage. If inserted a hard core of material, like a piece of PVP pipe, down a bore in the center that would work quite well. Other than that, I would see if you could get some thinner materials that you could strap along the outside of the foam like a splint and conceal that beneath the cheesecloth and Bondo. Regardless, such a is easier to plan into the design of a prop from the begining than it is to impliment it after the primary construction has finished, so I am not sure how much help that will be.
