A shot of acetone or MEK will probably thin that old dope just fine.
However, I'd be surprised to see nitrate go over the rustoleum primer successfully. There aren't many non epoxy coatings that will stand up to the solvents used in nitrate dope. Since you've primed it with that spray can primer, you're pretty much committed to staying with that type of paint until you're ready to clear coat. You will probably just have to accept having some of the fabric weave show in your final finish. That's not so bad- it has its own charm IMHO. You can get rid of the seams with glazing putty and sand them away.
Next time you do a fabric job, you'll know more and can plan for the results you want.