I use Acetone or Lacquer thinner for clean up as the MEK is more expensive.

Iv'e been using the substitute MEK without much of an issue.
I never covered and airplane before so what do I know? It's a PITA either way.

My paint shop cannot source any pure MEK atm so I'll make do. I have one gallon pure MEK in reserve if needed.
I did send an e-mail to the Poly-Fiber folks for clarification.