There are people a lot more knowledgeable than me on here, but my understanding is that: epoxy cures by chemical reaction, accelerated by heat, and produces heat as it reacts. So if there is a tiny bit of starting to cure epoxy in the mix, it isn't going to "pollute" the rest (i.e., make it cure faster) -- and once it is dilluted, I would assume the reaction would slow down anyway. Obviously, if a pot is actually going off (heating up) then I wouldn't mix more into it, but when glassing, the process involves getting the epoxy out of the pot and onto the glass as soon as possible, so there is usually very little left in the pot after only a few minutes (and once there is barely any left, it should be reacting pretty slowly anyway). You don't want to waste epoxy anyway, so should have scraped it pretty clean with a squeegee anyway! Maybe my climate helps, but I've only ever had epoxy start to kick when filleting anyway (which seems easiest to do using a pastry bag, but that necessarily means there is a big batch of mixed epoxy in the bag as you work).