Can I use oil-based paint over varnished bathroom cabinets without first using KILZ?

Yes you can. But you should use a product called ';NO-SAND'; just before you start paint on the existing varnish so that it will ';Soften'; the surface of the varnish and allow the paint to stick to the surface. But try this in a TEST area first so you can see if there will be a reaction to the paint such as wrinkling.Can I use oil-based paint over varnished bathroom cabinets without first using KILZ?
You can, if you sand the finish to give the paint something to hold onto first. Personally, I hate to sand; I'd rather paint two coats than sand. Quite frankly, there's a good chance you'll wind up painting at least two coats anyway, without a primer; depending on how dark your cabinets are, you're probably not going to get the coverage you want. A primer will not only help the paint adhere to the varnish, it really makes a difference to the final appearance. Make sure, in either case, you clean the surface well. I also suggest you use BIN or XIM, rather than KILZ. I think they have better sticking power. And do yourself a favor and remove hardware and cabinet doors -- it's much easier to paint the doors separately, and you get a much cleaner job if you don't try to paint around the hinges and pulls.Can I use oil-based paint over varnished bathroom cabinets without first using KILZ?
My motto is ';when in doubt, use primer.'; Why take the chance of ruining the cabinets and creating more work for yourself? And Kilz is good stuff.
No, and your cabinets will also need a little prep work before you get started. Sand lightly, then prime, then paint.
I wouldn't, I would probably sand first, at least try it in hidden area first!
I think it will crackle. Take the time to do the proper prep work, you'll be glad you did. It will take much longer to fix a mess.