I would like to build a dip tank to soak 6-12 shafts at a time.   I am using thinned spar varnish.   PVC is an obvious suggestion but not sure how compatible it would be with long term filling it with spar varnish.  I have a big dipper tank but it only holds 3 shafts at a time.
Any suggestions?

Go to home depot and check out the plumbing department. Also you can go to a muffler shop that bend their own exhaust pipe and get a 4" piece of aluminized exhaust pipe(they come in 8-10-12' sections).Get a nice 6x6 squared piece of flat steel and for a few bucks they can weld it to the bottom to seal it.

Home depot, plumbing supply

I like the muffler pipe suggestion.  PVC would be easy but still not sure if it would be compatible for long term oil storage.

PVC is fine. I have dip tanks for building bamboo rods that I made from PVC that are 6 years old.