I have a motorcycle gas tank that is about 4 years old. I recently had some welding modification work done to it, so it sat unprotected without gas in it for about 2 months. As a result, it got flash rust inside it. Its a light coat, but otherwise the tank is perfect. I am sending off to painter and want to take care of the existing rust inside first.

Important thing here is that I do not want to use the rust seal because I don't want it to fail in future. I purchased some aqua clean and rust blast and plan on using that. Any tips on what to coat inside of tank with after using these product to prevent flash rust for say, maybe 4 weeks, while I get it painted and put back together? Maybe a light coat of oil or WD40?

Would it be better to send to painter first and use rust blast immediately before putting bike back together and filling with gas? I would be worried about damaging paint that way, but if that's better I might do that instead.

It would be best to do the RustBlast step after the painter which is right before you fill up the tank with fuel. If you get a little bit of the RustBlast on the paint, all you have to do it wipe it off quickly and the RustBlast will not damage the paint. You can also blow air through the tank with a shop vac (or similar) after rinsing out the RustBlast to minimize any rust oxidation.