T&T varnish oil will work just fine on top of polyurethane (Permalyn) as long as the first finish isn't thick and glossy. You can also seal the wood with shellac as a first coat over the stain. However, for best figure, put your linseed oil finish on FIRST, then worry about the sealing finishes. If you don't want it glossy, just keep up with the T&T. I don't care what the instructions say, it does not dry in 24 hours, especially below 80F. It takes mine about three weeks per very thin, hand-rubbed coat at 80-90F and 50% RH to fully cure.
You want to put the T&T on, let it soak for a few minutes, rub it vigorously to heat it and re-distribute it where it soaked in, let it sit another hour, and buff it "dry" with a clean rag. It takes about 4-6 coats by itself to really get a good finish going. If you want the grain filled you will need to use a varnish with a little more resin. Mixing Minwax Antique Oil finish about 60/40 with T&T will both add dryers to the oil, thin the oil, and add an additional load of resins to it that make a slightly harder finish and bump it from matte to satin in gloss.
For more waterproofing, you can build PURE tung oil on top of the first coat of T&T varnish oil and the grain will have more depth than with tung oil alone.
