As a welder and someone looking at putting a new brewing set-up together, I'm in a similar position to you. My TIG skills aren't great and the stuff I do at work doesn't need to be sanitary, but I know enough that I can share my somewhat qualified (though not expert) opinions.
kladue suggested a couple of products as an alternative to back purging. I haven't used Superior 49 before, but I have used Solar Flux. It works fine and I was considering using it, but their
website recommends back purging if you're doing anything food-related. That's not to say it won't work; their warning may be more a matter of liability, and if you can wire brush the residue off after welding you might be ok. However, if you're going to the trouble of fabricating and welding your own equipment, you might as well take the time to properly back-purge.
I can't really disagree with any of the comments about grinding your welds smooth (and I've ground more than my fair share of welds smooth at work). However, rather than planning on laying down a big weld and grinding it smooth afterward, I would concentrate on things that will help you get a nice weld that doesn't require any grinding (like practicing your TIG, along with getting a nice fit and cleaning your pieces really well). Along the same lines of my comment about back purging, if you're going to do it, do it properly from the beginning (or at least try your best). From your posts, you're obviously either a welder by trade, or you work with welders, so getting a sound weld shouldn't be too difficult. With back-purging, practice and good fit, you should be able to get proper penetration and have a nice bead inside and out.
I'm not trying to look down on anybody who talked about grinding after welding, and it probably would be a good idea to stock up on some of those abrasives mentioned earlier. But I find that whenever I do anything that has to turn out "nice", I would much rather spend extra time
before-hand to make the job as easy as possible, rather than try to clean up a mess
afterwards. Again, just my qualified, but not expert, opinion.