never apply armaflex directly on the PCB, especially if you don't need to go cold there is no need to do so.
I mean, for -50° C or so you could just stick with a bit of eraser and paper IMHO.
if you need to go real cold armaflex tape on the back of the mobo could be a good idea, but before applying it I would cover the entire PCB with some kind of electrical tape so when you have to get off the armaflex the PCB will be clean.
armaflex glue is a pita to remove.