I have had some good surprise also Huvius. Bought old guns at various gun shows that sellers were too lazy to clean, and ended up after a lot of extensive cleaning having a very nice bore.
Purchased a Ballard Pacific recently that the gun shop never lifted a finger to try and clean. It looked horrendous with some sort of brown gunk all over the metal! I got it for a song, and spent 6 hours stripping the entire gun down to it's base parts and cleaning it. It took acetone to break the gunk loose, but underneath was a very nice Ballard Pacific!
Before, with one spot cleaned to read the caliber (.40-85):
After, entirely cleaned:
