We just finished our 3rd bellymask after using a kit of plaster rolls. We cover it with spackling paste/Gesso several times and sand it smooth. Im just wondering if you know of anything that can make it even STRONGER though? Perhaps something I can coat on the inside that would make it extra durable? Our 1st mask made 5 years ago has a few chips on the edge.

Any help/advice would be great. Thanks!!
Erica

Don't know if this will be all that helpful.
Rather than using spackling/gesso you could use plaster mixed with white glue. I'm told that adding white glue to plaster will strengthen it. Add enough (50-50 with water) and it can even become somewhat flexible. You would have to experiment with it as I haven't actually tried it. You could also add fiberous materials, like fiberglass, to the plaster.
Of course, if you're just looking for harder, hydrocal works about the same as plaster but ends up being much harder.
As far as coating the interior surface goes the plaster will soak up just about anything liquid when it's dry. I'm not overly fond of this method as much detail can be lost. If you should decide to go this route I would suggest trying Decoupage resin as it takes several hours to set up. This gives it plenty of time to soak in thoroughly. It's main drawback, besides the fumes, is it needs to be measured precisely or it'll remain sticky for a long time.
The easier, more user friendly, way to coat the interior would be to brush on well thinned down white glue. I would probably brush it on till it was saturated and then let it dry. No second coats.