Unfortunately Robo is right. If the density of bees gets too high (nuc too strong), the bees will start carving out foam to make room for more comb. I’ve got boxes where they completely chewed out a side to build a new comb. To prevent this problem you either have to face the interior surface with something, of you have to be very diligent watching the hives and splitting before they get too strong. The bees overwhelm small nucs pretty quickly.
I’ve got some 30 unfaced foam nucs in use (see photos above) and they do work well, but once it’s time to replace them, the next ones will be faced with something inside. I also used thinner foam for my mating nucs since I didn’t intend to overwinter them at the time. They don’t need a lot of insulation for summer use. My thinking is, if you want to overwinter a mating nuc that is 4 mediums or less, then it’s cheaper and more reliable to go with electric heat.
I would bet my last dollar that coating the interior foam with bees wax wouldn’t stop them. The bees harvest bee wax from frames/foundation all the time. Thick paint, or poly, would slow them down a bit.