I would put a spar in a wing of that span or run carbon fiber cap strips along the top and bottom of the wing.
If you are painting directly to the foam make sure the paint you are using is foam safe. Most, but not all, of Testor's spray paint are foam safe. I've found that the metallic Testors paint will eat foam a bit. Not nearly as bad as your typical rattle can paint, though. Apply lightly with multiple coats.
XPS is very prone to getting dings and scratches, so you should consider covering your wing. I like covering my wings with 0.56 oz/yd fiberglass cloth and applying it with Minwax Polycrylic instead of epoxy resin. Polycrylic is easier to work with and MUCH less expensive. If you aren't into doing that, you can can cover the wing with document laminate, which can be bought at office supply stores. I've painted on laminate film with an airbrush but never with a rattle can. You could paint the foam, then cover. A lot of people use colored packing tape to cover wings, too. Lots of options!