Thinning epoxy slightly with MEK or acetone is not a problem for non-structural purposes like fuel-proofing a firewall. I don't think any of the posters suggested for structural applications. I don't know why you would need a Masters in Chemistry to do this without extensive testing/wasting of product. Add a few drops, mix it in, if its thin enough to brush on, have at it. If not, add a few more drops.