I've stripped/sealed some 300 tanks over the last
15 years. It's always been virtually impossible to
install Kreen such that it does not fail, and quite
difficult to remove when it does...
MEK works best, but that's nasty, dangerous stuff to
use, and hard to dispose of once used... It'd be nice
if someone(s) sued Kreem out of business.
I use Por 15, but like getting a good paint job, proper
preparation is vital, or Por 15 can fail too.
There are many types of plastics; some don't deal well
with gasoline and/or alcohol: Google "Ducati tank swelling".