You can thin epoxy a couple of ways.

Add a drop or two of alcohol after it's mixed. I buy denatured alcohol (contains 99+% alcohol) at the home improvement store. Drug store isopropyl alcohol can be used if it doesn't have too much water in it. Around here you can buy 70% or 90+% ispopropyl. The other 30%, or <10%, is water. Everyone has an opinion on this. Mine is that you don't want to use the kind with 30% water. Be careful about adding too much. A tiny bit makes a huge difference after you stir it up.

Heat is the other way to thin it. A blast from a heat gun can make it as thin as water, but it also makes it cure faster.

In either case, use 30 minute, or slower, epoxy. The 6 minute stuff will be a rock before you know what's happening.

The experienced builders can give better advice but I'm not sure you want to thin the epoxy on the wing joint fiberglass. Thinning it for fuelproofing is fine but thinning it on a structural joint might weaken the joint.