Thinning with alcohol does all of the above.
Thinning with acetone (the product developed to thin epoxy) doesn't.

But there is an excellent suggestion just made...................

If you wish for thinner epoxy, then try the ones that're labeled as surfacing resins. I use the stuff all the time for fuelproofing and strengthening the insides of fuselages from inside the firewall back. Works great. Brushes on excellently. And it can also be used structurally (understanding that it's not going to give the same results as regular resins) with fiberglass cloth, and can be layered as long as the layers are done at the same time. Great stuff.

Just modified a P-47 to have a round tunnel through the firewall into the next bulkhead. I wanted to completely enclose the muffler within that big old nose. I used a mailing tube that was about the right size. Cut the holes in the firewall and bulkhead to match the tube. Cut the tube to match the length from hole to hole. CA's the tube into the hole, then painted it and all the raw wood with surfacing resin. It's as solid as anyone would wish. And was dead simple to do. The resin was especially simple and easy to do. If fuelproofs perfectly and turned the tube into a structural piece. There actually wasn't much support fore and aft in that area for the front of the wing's attachment to the fuselage. Now there's more.