Venice: Ooh, I'm modeling for that panel. Which basically means I'm letting my friend paint my arm and helping demo some troll grey. But I can basically tell you what you need to know now!
It's super important to seal your makeup. Seal seal seal! You never know if you'll touch someone's costume on accident and ruin it. Sealant comes in two stages: powdering and fixer spray.
For powder, you want to get a colorless setting powder. Not translucent! This is very important, because translucent usually has a skin tone, so it'll turn your grey pinker. Colorless usually looks white in the little container, and it's very very fine, so it doesn't effect the color of your paint at all. You either need a powder poof, or a powder/blush brush to apply it most efficiently.
For sealant, the easiest thing to find is either Ben Nye Final Seal or Kryalon Fixier Spray. They both come in little spray bottles.
If you live in Portland, you can get Ben Nye Final Seal and colorless setting powder from either Costume Avenue (in Beaverton) or Hollywood Lights (on McLoughlin Blvd). You can get any Kryalon stuff from Hollywood Lights.
Also, practice applying your makeup before con! Full coverage of face, neck and hands will take you time. The first time you do it will take you anywhere from one to two hours. If you can get a couple practice sessions in and you start to get to know your paint, you might be able to cut the whole process down to one hour.
If, for whatever reason, you can't get sealant in time for con, I'll be at the meets with a can of hairspray. The nice thing about hairspray is that's basically gentle aerosolized glue. It's not that great a replacement for actual body paint-intended sealing spray, but it's a good stopgap measure to keep you from getting paint on anyone.
