Any acrylic clear coat short of artist quality brush on clear coats (which would prove expensive) are too thin and putting a solvent based product over it will risk burn through. Good old latex paint brushed on or applied heavy with spraying is the way to go.
You arn't supposed to but, I have sprayed tombstones with latex paint through an automotive spray gun then cleaned it up good with solvent. It is best to just brush it on with a dark color as your base.
Using fiberglass resin as a hard coat can shatter like glass once cured but, things like 2 part polyurethane like what you would pour over a decorated table top are more elastic and less caustic to handle. It can also be tinted if you desire with universal paint tints and be the final finish.
The krylon if put on too thick (trying to make it do something) will not cure properly and remain sticky forever and cloud up blocking the possible under finish of any work.
If you want to go to pro level finishes, flatners are available for automotive clear coats these days which happen to work in anything, any medium. They are dry powders like pearls.
With styrofoam I would stay away from anything in a spray can even if it is supposed to be acrylic. These have solvents in them just to stay mixed and dispense with air without clogging. Unless of course you want it to be eaten. Sometimes wrong is the look you are going for.
Another fabulous post from the U.S.Department of Wild Imaginings, now in spectaclar stereo, sponsored by the Adhesives and Sealants Council, suggesting ways to stick things together since the 1800s. Not fabulous in a gay way. Your results may vary. Illinois residents add 8% sales tax. These posts have been made by professional post makers, do not try this type of posting on your own without extensive training, lovely assistants and a trusty clown horn.