I'd clean it out, and see if what remains dries. Either there's something wring with the tru oil, or there's a chemical reaction going on stopping it setting. Have you tried it without the pumice powder? There may be something in that that's not just pumice.

I keep getting areas on my spraying that just bubble up for no apparent reason. It's very frustrating but there must be something there that's causing it, even if it's only a very small impurity that acts as a catalyst.