Here's one problem that has been overlooked as these Briggs tank has a reservoir cup that can develop pin holes. The main tank is fairly easy to coat, its this reservoir cup that is a pain to do as there is limited spacing between and the carb tower.
Also many of three tanks have plastic and foam baffle materials especially those on garden tillers.
Right about the reservoir cup.
I hinted about the reservoir cup and fuel level in post #18. If reservoir cup has leakage (or the long tube is not pumping fuel the engine will need the fuel level in the tank above the level of the short pickup tube.
If I encounter one of these type carb acting weird
I fill the tank full of gas and test. If it runs ok with such it's a hint that the issue is with the long pickup tube or reservoir pan pump area. Operating the engine with a full tank will let you repair the carb later if the equipment is really needed at the immediate time.
Do not let the sealer coating build up in the bottom of the reservoir cup or the bottom of the carb short tube will get restricted/blocked when the carb is installed.
As mentioned some of the tanks have the foam baffle materials.
I've removed that foam baffle on some tanks and some will clean up good due to a good non-rusting type metal and others will not get clean, rust too heavy and tumbling will actually create pin holes. Just have to start tumbling and see what the results will be. I get surprises how well some of the small engine rusty tanks will get clean by tumbling and would live long time without being coated if proper fuel storage is used. Ethanol gas left in the tanks seems to speed up the rusting. If small engine metal tank is going to be stored for long time I add a 2 cycle mix to the tank to reduce chance of corrosion and rusting. The engine will usually operate ok when needed using a 50:1 2 cycle mix. (and carb pump diaphragms last longer)
Not a fun job but some of the vintage JUNK stuff such as tanks are NLA as new. (or priced out of sight)