BMatthews

For flat curves (as in single axis) curves there's nothing wrong with the paper to cover the foam. But you're right that the foam is too easy to sand with some areas which become compound curves. In such a case using glass cloth will replace the paper nicely and it'll lay on the foam more tightly.

The key to doing a smooth glass job is to not use too much bonding agent for actually sticking the cloth down. Go with the minimum you can get away with that still fully wets out the cloth. The idea is that you don't want to use so much that the cloth floats up off the foam and begins to slide around easily. Save filling the weave for the second and third coatings.

The cloth will only achieve a rigid stressed skin shape if you use a bonding agent that is stiff as well. This is why polyester resin and epoxy resins are so good with fiberglass. They lock the weave together more solidly. Note however that you must use epoxy resin for foam as the polyester resin will dissolve the foam.

Lots of folks use things like water based polyurethane (WBPU) or the Minwax Polycrylic (which is some other concoction) The WBPU isn't too bad but the Polycrylic is like a coating of elastic it's so flexible. Using WBPU to glue the cloth to the foam may well result in an adequite level of stiffness. Using epoxy laminating resin for the first bonding coat certainly will do the job.

Note that there's a difference in "stiffness" or "rigidity" and ultimate "strength". You need "strength" to resist hard landings and high G loads. But if you lack "stiffness" or "rigidity" the airframe is too flexible and will not hold the wings and tail in correct alignment. This is why I'm bad mouthing the Minwax Polycrylic due to the rubberiness of it. It lacks the stiffness needed to let the glass fibers actually lock together and become rigid enough.

You can fill the weave of the cloth with a number of things. But one that I'd suggest you try is to buy some of the very lightweight wall filler such as Red Devil. You'll know you found the right one when the container feels like it's empty. Thin this with some water until it's a soft and easily spread peanut butter like consistency. Use a small putty spreader to fill the cloth weave with a fairly generous coating. Allow to fully dry then sand away the excess. It sands VERY easily so don't get carried away. To harden up this filler apply a top coat of WBPU or solvent/oil based PU to soak into the filler. If you still find that there's some small amount of cloth weave showing at this point you can likely spray on a couple of coats of automotive sandable primer. Now this stuff is heavy so be prepared to wet sand most of it off down to the point where it looks almost transparent but not quite. By now you should have a pretty nice surface for painting.

If using products which need to dry instead of epoxy you really want to be patient with the drying of each coat. It'll go a long way to avoiding the cloth weave telegraphing through later if you let each product fully dry and finish shrinking.

Depending on the thickness of the foam you leave you likely don't need an internal box of plywood or even balsa other than at the nose and back to the trailing edge of the wing. The foam and glass skin will easily take the flight loads provided you use a decently heavy grade of cloth. Or if you use two layers of cloth to achieve a skin suitable for a 50cc engine. The doorskin "short box" then provides the vibration path for the engine's firewall as well as a solid structure to use for mounting the radio gear. But it's main purpose is to give the engine vibration carrying firewall and forward box a bigger "glue joint" to pass the vibes into the foam. If you "just" use a firewall to do this there's not enough foam to wood surface area. Even with the glass cloth to help you'd likely still find that the foam around the firewall becomes loose with time as the foam fails and crumbles. A short "semi" box internally that connects solidly to the firewall will go a huge way to avoiding this sort of issue. It also gives you a nice solid place to mount the landing gear and wing hold downs. Again these point loads don't do well in "just foam" even if it has patches of fiberglass at those points.