You're in the right place
I'd urge you to go with either liquid latex or exterior house paint. Paint the first few layers on very
lightly. Otherwise, it just absorbs into the foam like a sponge and goes to waste. It takes 10-20 layers to get a smooth surface. Let each layer fully dry before proceeding or the surface will stay mushy for ages. If you go with liquid latex, apply paint over the final layer before moving the prop around; dried liquid latex really likes to stick to itself. Either way, the result is smooth, glossy, flexible, and durable.
Fiberglass resin technically works, but it is more expensive, much messier to work with, smellier, more temperamental, and you just don't need something that rigid unless you actually need functional armor for sparring.