Man I feel this in my bones because I have been there way too many times. Getting called back out to the same house for round two on a glazed pipe is the worst, especially when you already told the customer it was done. Nobody wants to eat that second trip and nobody wants to explain why. I think you are right that some of it is just the heat doing the work though, those logs burn hot enough to dry the glaze out and crack it loose. But whatever the reason, if it gets the job done in one visit instead of two I am keeping one on the truck too. As far as charging full rate, you did the full job and solved the problem, so yeah I would bill it the same. The customer is paying for a clean pipe and a safe stove, not for how many trips it took you to get there.

Thought avionics tools were low key but that airport story changed my mind.