I'm looking to use aluminum in a heat transfer type purpose.  I would like to use "roll flashing" (.019" thick) typically sold in big box lumber stores.  I can actually get it cheaper if painted (both sides) rather than mill (raw) finish.  This will NOT be used in a solar type situation where obviously black would be more conducive to drawing more heat.  This will be used in an under floor situation. Just wondering if the paint has any real impact on the heat transfer rate compared to a mill finish???

No. But unless you are a highly skilled sheet metal technician, what you have in mind will likely not work as imagine.

Oh, ye of little faith. (And .... No, I never even took a shop class!)

I have great faith, but little hope I'm afraid.

Please report your results, whatever the outcome.

And, all the best!

Thanks!