If it is dry go ahead, I tend to wet sand with 1k grit if there are any flaws but if it looks good go ahead. Go easy with the clear. I would definitely test the clear out on something before you go live on the tank. But since you are getting work done on the tank I would hold off because you will most likely have to wait anyways because it may get scuffed.
Once the welding is done can you reinforce it as a precaution? I probably would just as a safety measure but I have never delt with tanks like this. Would not be fun to get it all done and have a failure. Perhaps a weld then a composite applied to the inside. Hmmm I really want this to work out for you.
You need Erling!