I just mixed polyurethane with formby's tung oil finish and applied to red oak. This was following two coats of plain old formbys. I wanted to thin the poly but had no mineral spirits on hand and thought formbys is probably thinned with mineral spirits so how bout i just use that. Now im trying to find out WHAT THE HECK IS IN FORMBY'S, cant find anything specific on the internet. Any knowledge on formbys eapecially mixing it with poly would be much appreciated.