[Dave in MN]

Good thing you only have the tank in primer. Cleaning and coating system components can be damaging to top coat paint.
There are solutions available to pour in the tank and "slosh" around. The solutions work with limitations. If there is a build-up of rust, you may need to cut open the tank to properly clean it.
Two of three cars I own have had the "solution" method of cleaning and subsequent coating applied. The coating is holding up without any flaking as evidenced by a clean internal filter above the shutoff and at the carburetor.
I used a cleaner, metal etch and sealer marketed by Bill Hirsch. I just checked his website, the "AUTOMOTIVE FUEL TANK REPAIR KIT" is exactly what I used in both Model A's.
Note: With one of the tanks, I placed two handful's of 1/4" steel hex nuts in the tank and shook the tank in a multitude of orientations to cause the nuts to strike the inside surfaces. I was able to get lots of debris loose with this method. I was 25 years younger when I did this and it was hard then. The tanks are not light when shaking them.
Hope you have success...Good Day!