Never tried MEK... that stuff is nasty. I have some around for reducing auto, er I mean airplane paints.

In general, anything liquid added to expoy will weaken it. MEK will evaporate really fast, but I guarantee some will still get trapped in the epoxy, which will weaken it.

The best way I have found to thin is with heat. Warm up the bottles before you mix in a water bath. You can also warm the epoxy after it is applied with a heat gun. The heat will make the expoy set quicker. I've heard that added heat can also weaken the cure, but as long as you keep the temp reasonable, say 120F or under I think you'll be fine.

If you are trying to fuel proof, you can also use heat, but since strength isn't as important, you can get a bit hotter. I'll pour, slather, brush in, etc the epoxy. Hit it with the heat gun, then use a playing card to scrape off the excess. Makes for a pretty thing and light weight fuel proofing. Only way I know of to get thinner/lighter is to use dopes. I don't like the dopes because they never really cure and remain tacky for a long time.