Is there any solution that will soften cured proseal / flamemaster enough that it can be somewhat easily removed? I'm talking about the interior of a fuel tank, where it is pretty thick, not a few small drops...
usually people make a sharp edge on a putty knife and slide that under the proseal. I don't know exactly what part of the tank you are having problems but if you can get that in there that would be the fastest
I had to remove some and found a common wood chisel to remove the bulk worked best, then I used my little scotchbrite wheel in a die grinder to remove the remaining traces, and make the metal like new again.
MEK and a hard plastic scraper works for 890 Proseal. I used to use a cut up LearJet windshield, which is plastic, and it worked great for removing proseal from fuel panels.
I am not familiar with the flamemaster.
MEK as you know is not real "healthy" when vapors are inhaled or if you let it be absorbed into the skin.