Someone did it straight on the Biplane Forum.
I used the System Three Clear Coat which seems to be already thinned T-88. I guess it depends on the coverage needed. A little spot you could thin with denatured alcohol. I do that sometimes with model planes, but I would have to consider the actual job if I was to do it on a real plane. I would not want to glue to it if I thinned it. Pure varnish only. Repair only. Full varnish job, I would get a different product.
I used acetone with the MGS L285 system, but you can also use denatured alcohol. I "varnished" both sides of my ply sheets (0.8 to 1.5mm) to prevent the buckling and wobbly looks with changing humidity (I live in a tropical/sub-tropical climate with long, wet and hot summers. Apply it thinned but in generous quantities to allow good penetration and still have a decent volume of epoxy. It also spreads a lot easier, just keep moving it around (paint brush) until it starts to gel.
The positive side effect was that the ply sheets are quite a bit a stiffer and more resistant to deformation when touching (sanding). Just be aware that most epoxy is not very UV resistant and exposed areas need to be protected with marine varnish or paint.
Epoxy kicks most solvents out of solution when it cures, giving a solvent filled foam. That will be a little porous. I'd rather use heat. Heating the workpiece seems like a good way to have sane pot life and still let it go on thin.