I live in western WA state, where it typically rains from mid Sept on through the winter. What I did for ground breakers was "mache" with latex house paint, not glue or flour or water. I'd build my frames from scrap wood, pvc, chicken wire, bucky skulls (or whatever plastic skulls), sometimes padding with that super thin sheet foam they wrap pre-hung doors with for shipping, as I scored a pile of it once...all that stuff is waterproof except the wood base or sticks, which I'd just coat a couple times with house paint before I skinned it. For hands I'd use coat hanger wire or those thin wires they use to get fiberglass house insulation to stay up between floor joists (simpler than fiddling with hangers, and you get about a million to a box, it seems), then just brown masking tape over to give the mache something to grab onto.
I liked mache-ing with paper towels and spider web material mostly, sometimes latex gloves (cut a piece and superglue it to your skull for a "cheek" or other scrap of skin, then poke a hole or two for that ripped skin look..beats the heck out of dealing with fresh latex and works just as well, and cheap enough), but you could use newspaper or any kind of paper or cardboard, tissue paper, or whatever you like that will soak up the paint. Anyway, dip straight into the paint, squeeze off the excess between your fingers, slap it on...the usual mache rules for thin coats and letting it dry apply here. When your masterpiece is done, if it's a lighter color, you can stain it for a corpsed look. The smallest can of minwax stain will do many groundbreakers, just slather it on with a disposable chip brush, getting it down in the deep places, give it a few minutes if you want, then wipe off the high points with a paper towel or scrap of old t-shirt. Let it dry, voila, done. I've left these out in the daily rains the whole month of October, they held up just fine, and if it was especially wet that year, I'd just let them sit on the work bench with a fan on them overnight, then let them sit a few more days before packing them away, to make sure there was no moisture (mostly it was just the wooden base that needed attention, if anything did.) When we moved to our current house about 8 years ago, we stopped having TOTs (too rural), so if I make something now, I donate it to a charity haunt or home haunters, otherwise my shop would be overflowing, lol.
If you're desperate for paint and have no luck at the mistints shelf, America's Finest brand is cheap for flat exterior house paint, it's at Home Depot, $12 a gallon where I live, and it's a pretty good paint...they also make a pretty good spray paint for pvc fencing and such. A gallon of black and a gallon of white flat house paint goes a long way for a haunter: you can coat with black, dry brush with white or mix up a little gray and drybrush for things like tombstones and "stone" look items, black for hiding support structures/frames/bases. You can almost always find something on the mistints shelf for mache work, the first few layers of which are hidden anyway. Another source for paint: stop by a local house painter or call around...they often have partial buckets of paint leftover from jobs, they'll happily give it to you to avoid having to fiddle with disposal...mix it all together and it will turn gray or brown. I've done the monster mud thing, too, with just house paint and drywall mud...those stood up fine despite the rain. Unless it's an electronic prop, I wouldn't worry about it getting rained on, just make sure it's dry before you stick it in a box, and make sure your storage area is dry and warm enough to keep things dry the other 11 months of the year. I've used plenty of props with just raw material like clothing or bed sheets for dressing them...it's fine as long as you dry it thoroughly before you store it.
