Tank studs. Clean the tank studs with a wire brush or wire wheel:

Tank.. Soak the tank in Marine Clean overnight. The method I used was to fill the tank just over half full with a 50-50 mixture of hot water and Marine Clean, then stand it on the end with the fuel tap down for half of the time, then with the fuel tap up for the remaining time. The warmer the mixture, the better it works. But be careful with Marine Clean, it reacts with aluminium. Therefore, if your gas cap has an aluminium vent cap screw, it should not remain in Marine Clean for long. I had four tanks, three had aluminium vent cap screws and one had brass. I used only the one with brass during the soaking operation. For aluminium, I would fill the tank 100 percent full and leave it level with the cap off. I find that the Marine clean will dissolve it all except possibly for some solid residue that sometimes remains.

The tank on the far left had a slow leak and needed to be positioned over a glass to catch the drops:
When done soaking, wash out with warm water and shake well and flush. Wash several times until no more debris is produced. If you can reach into the tank with a wooden dowell, scrape free any residue that did not dissolve. Then, you can put some Naval Jelly or other "instant-acting" tarnish removers. All of these products will have a mild acid that will dissolve the green corrosion residue in the tank. I find this step not so important as a little green color adds character! The dents in the tank also add character, so why should the character only run skin-deep?

Fuel tap filter. There are many problems you can encounter with the fuel tap filter.

The filters above are all present and in fairly good shape. The one on the far left needed Naval Jelly treatment which slowly dissolved the green corrosion. The one on the far right needed to be cut off, shortened, and epoxied back in place. The one next to it was crushed, but with a little care, was made almost round and was used as-is.

Either it fell off at some point, it is crushed out of shape, or blocked with residue. Assuming you still have it and it is not destroyed, Clean it out with Marine Clean to remove oil and gas residue and Naval Jelly to remove tank corosion residue. Dry it out and look through it. You should be able to see (with a magnifying glass if necessary) the fine texture of the wire mesh and can see if any of the filter is still blocked. If is had been come off the fuel tap, clean out the recess in the fuel tap so only bare metal is left, then epoxy it back into place, making sure you don't use much epoxy. Be very careful you don't block any of the mesh. Do not solder it!! I have had good luck with West system Epoxy, but any good epoxy should work. If the tap filter is missing or destroyed, the wire mesh filters are no longer available but you can order a plastic one from Sheridan Marine (P/N 1307F) which can be cut down and epoxied in place.