I saw my dad use a heat gun on my piano before. It had been stored in the shop out back, and we only turned on the air-conditioner and heater while we were in there, so the air would get pretty humid sometimes.
Some of the keys were sticking. When I would press them down, they would just remain halfway pressed for a long time, and this made it very hard to play a song.
He opened up the top and pointed the heat gun at the wooden part of the keys that were sticking. He knew what he was doing, so there wasn't any danger of him damaging the wood. This worked for a long time, but it has to be done every few months to un-stick them.