It probably would be a good idea to put the resin in clean plastic container/s and do a small test patch before you use it for real. Back in the day cut down milk cartons were used but I think that was with polyester resin. I'm far from an expert in this area so I only use clear plastic cups or well washed yogurt or cottage cheese tubs for epoxy work.

my gut says that you are probably just fine.  i have, in the past,  worked with obviously waxed cups and had no problem.  that said, would not recommend it.  can see potential problems if you mixed hardener and resin in a wax coated cup and generate enough heat through the epoxy curing reaction to melt the wax/pull it into the epoxy mix.   but everything i did did not involve anything more than room temperature.

that said, use cheap plastic cups and you don't have to worry about it.  if you are very cautious, make a test batch of epoxy with the stuff you stored in the wax cup to confirm it still cures with no unanticipated problems.  keep the batch seperate from the new stuff so you can trace any problem back to its source.

   OK, I'll tip them upside down overnight into plastic cups.

thanks!