If the shop did clean the tank then you are good to go.

Coating the tank with after market products can cause problems if the coating does not adhere completely to the inner metal. Sometimes it is a necessary evil but I'd avoid it if at all possible.

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now