Well, it sounds like you've already done the hard part.....welding it. I was gona chime in on that, but no need now.
Grinding the can off is a pain, but if you have a mill/face mill, it's just tedious.
It's very rare that anyone gets a "can" to peel off cleanly. If the rust thing isn't working, and you have an oxy/acetylene torch, you can cover the can's interior with acetylene soot, but that can be hit-n-miss too. I shy away for the whiteout thing....only because it's about $40 worth of whiteout to line one of my cans....and it doesn't work any better than anything else.
Unless I intend to clean all the can off a billet and use in in a composite, I tend to build my cans in such a manner, that I can leave it a 1/4" or so larger than the finished width I want, and saw slices off the can/billet. You do have to remember how thing are oriented, and as I always say, I err on the side of leaving things too large....because I can always grind it off/down..... but if its too small, I can never put anything back.
Over the years I have built my shop tools, to handle/make very large cans..... with the idea being that if I start with a 4-5" square can, by the time it's down to 1 1/4- 1 1/2", that can is really thin......and then, as long as I've been careful to keep things square, it's usually a single pass per side on the face mill.....and it's clean. The other benefit being that I get a large number off slices/billets off the bigger cans.
Make sure you post some pics!!!