what are you seeling a latex mask or a hockey? if you seeling a hockey just use a clear spray, if your painting a latex mask, you shouldn\'t have to seel it, unless you need something to be shiny then use perma wet, but if your using a paint that wont stick to the mask then your useing the wrong paint.
It depends on what you are going for, I see you put you need it to flex so I assume latex. If you want buy some gloss or matte finish in a bottle then take half of it out then use your liquid latex and add that in, it will flex & no cracking or problems what so ever. You can actually coat the whole mask with the matte finish & then finish up with the gloss for any wet spots or sections that you want more shiny. Works just like Perma wet & hell of alot cheaper to do. If you have any problems running it threw your airbrush just thin it down abit. Hope this helps a tad.