To add to Dave's description - I have some propionate pellets here, only used it a few times and seems to be a good sealer, as a top coat it's a bit brittle--
A bit (a lot really) of research many years back told me it is/ was used in lot's of plastic apps, including those clear yellow chisel handles we use (very tough), sunglass and eye glass frames, for it's toughness and the weird property of always feeling like it is at body temperature when you put a pair of glasses on, (never noticed this until I read it)-----also used for spectacle lenses , so here's what seems to be the ideal plastic for our purposes.
But the best bit-------
Beg, steal, or bludge all the old sets of those cheap reading glasses from your friends (make sure they are not GLASS), pop the lenses out of the frames and place them on a piece of heavy cloth and cover with some more cloth, find a jar with lid, THEN bash the crap out of them (lenses) until you have reasonably sized pieces and tip them in the jar, then add some GOOD QUALITY lacquer thinner (about 4 :1 mix of - thinners : lenses), shake it up and leave it for a few hours and shake it again and again for about 48 hrs, if it looks a bit thick add some more thinners and you will have a jar of' "Prop"- thin it to your requirement and don't let it thicken too much, over time (weeks / months) it will turn into a clear solid lump, so keep an eye on it.
Also- DON'T just dump the lenses in the thinners without crushing them, they will stick together and it will take a week or two to dissolve.
Over time it seems to yellow a bit, just like all the other 'upmarket' clear coats we use.
Pete