I made the piece with the two part clear coating and casting resin 'Amazing Clear Cast'. It's my first time using resin so it ended up with bubbles and dirt embedded in the top surface. I asked a friend to polish it since she polishes cars for a living. It looks great, but unfortunately when I got it back my cast was no longer flat. I think it may have gotten too hot during the polishing process and is now warped and won't sit flat (it was level and even like a drink coaster).
Is there any way to save it? Can I heat it up at a low temperature and let it cool on a flat surface to remove the warp? Will it break down the material or make it more brittle? And if it is safe to heat at low temperatures, will the painted design encased in it be damaged or distorted?
It's a gift for a friend and I really don't want to start over as this is 3 months in the making. I poured the base layer, let it cure completely (for maybe a week to be safe since it's cold), painted the design on top of the base with plain-old acrylic paint and let the paint dry completely, then I poured another thin layer of the same type of resin over the whole piece to protect it so that the design is sandwiched in the middle like it's floating.
Any additional heat should not cause any further damage. I would use something like a hair dryer, something that you have some control over as far as the heat goes, to straighten out the piece. I would place the bottom side up/top side down (to protect the visible side) on a flat surface and place a weight of some sort on the piece, that once it gets heated up enough, will force it to flatten out. I use a speaker magnet that is aprox. 3" in dia. and a couple inches thick for fixes like this as it is a solid piece with more than enough weight to get the job done.
Brian's right - additional heat and a weight on it does straighten the Clear out when needed (I've done it) and the hair dryer is the way to go most times with thin pieces. You can also use an oven set no higher than 275 for 15 minutes at a time then take it out and put the weight on it. Keep an eye on it and you might have to do it a couple times, letting it cool off completely in between warmings. Be aware that you might get some fumes released off it so try not to breath those in. Personally, I go for the hair dryer first as I can control it more while the oven does an "all over" heat. Also if it's your kitchen oven you'll need to air that out for quite a while after heating it so hair dryer is usually a better option unless you have a toaster oven just for these kinds of things.