Can you use tung oil on the epoxy's substrate?

No, tung oil will repel the epoxy's attempt to bond with the substrate.

If tung oil is used on a substrate and epoxy is poured on top, the epoxy will bead up like water on car wax.

Note: If you absolutely need to use tung oil we recommend the following method:

1. Wait a month after applying tung oil to your surface.
2. Apply two to three coats of oil-based polyurethane.
   1. Warning: Make sure every single inch of the surface is covered with the oil-based polyurethane, or you'll have issues later.
3. Apply the epoxy to the surface.