The guy said they wouldn't vat it. Just a guess, I'm thinking the fuel buildup in the tank might contaminate his vat. I brought the tank home rather than paying the people 200 bucks to do what I believe can be done at home. Mind you, this is not the tank which produced the rust in the filter of the OP.
The worst of the rust in at the inlet and extends just below the guard inside the neck. I used a flashlight and did the best possible to inspect the top, rear and front of the tank (as it would sit in the car) and found no rust except for the bottom in the location of the hole for the screen to mount. I suspect the bottom and neck may be the only spots with rust and it appears the rust in the bottom of the tank is surface rust.
Here's the photo of the rust residue on the bottom of the tank at the screen outlet:
Here's one of the inside of the neck:
The rust only extends a few inches past the camera view.
I can certainly strip the rust from the neck by dipping the neck into a bucket of something to dissolve the rust but I'm not sure what acid to use. I'm thinking a solution of citric acid and water but I don't think the concentration used to flush an engine cooling system is going to do much with the rust in the filling neck.
Any advise on how strong a concentration of citric to use?