What to do with filler neck

Here's the update after a few days soaking in Evaporust. FYI during the soak I took it out once to scrub it with a scotch-brite pad and rinse it, and then put it back in.

The thing looks much better than before but there's still an issue. See pics. I took pics with both flash on and off for different contrasts. The Evaporust left a green film coating on the outside and inside the filler where you put the gas pump. I was able to scotch-brite that off (most of it) while rinsing it in water.

The question I have now is, what to do about the yellow stuff at the end of the filler that goes into the gas tank? Before I used Evaporust that was heavily corroded a white-ish color. Now it's yellow. I tried to scrub inside the tube with a toothbrush and a wire toothbrush type of thing and neither did very much. I sprayed it down with brake parks cleaner and that didn't have an effect on it. Right now the whole thing is coated inside and out with WD-40 to stop any rust.

The yellow stuff only goes maybe 6-12" up inside the tube, beyond that it's clean. Any ideas? Should I just ignore it and move forward with using this filler neck? Should I try soaking the bottom of the filler in something more harsh like vinegar?

EDIT - just a little more info. After 24hrs of soaking I didn't see any difference beyond that. It soaked for 3 days. It wasn't like the yellow stuff was dissolving slowly or anything or else I would have let it soak for longer.

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