It's hard to get Monocote over Ultracote... it wants a higher temperature, so the Ultracote underneath will try to shrink away from the repair while you're trying to get it to all stay together. Whatever you repair with, nake sure you clean the existing covering well. Alcohol cleans it, but MEK or acetone is better.

The ding really should straighten out with the right heat. If not, try a little hotter til it behaves (with an iron if possible... it's a really easy way to melt a hole in the covering if you try it with a heat gun.)