The fuel tank on the passenger side of my truck has a corrosion problem. Not sure why or what's best to do about it. It still has fuel in the bottom but I don't dare switch to it because I'm worried about the rust particles that may get picked up.

At a minimum I think I need a new cap, seal and the filler column thing.



Looking for motivation/suggestions really. Can still drive about because the driver side is fine.

Can you buy a new one?

Yea seems to be available, £200 for the tank without a cap or filler though

Probably another £250 for a cap!
Guess if you don't want to use it, empty it out, clean it up , paint it and leave it.

Cheapest I've seen a cap is £111, ouch, might try scrubbing the rust off the one I've got.

I'd like to use it occasionally for longer trips. What causes it? Just not used often enough, condensation gets in?

Ethanol in petrol. Introduces water. Buy Esso supreme only. E5 Max, usually zero.

Plus tank isn't full, keep driving and using both tanks.

I get crud in Gigi's fuel from the metal tank. I may need to get it lined to protect from the Hydroscopic effects od Ethanol in petrol, which attracts water into the fuel.
I have addded a super powerful magnet to the bottom of tank, regularly clean the glass bowl pre fuel pump crud filter as seen below.

Also add some Reddex each fill as well as the lead replacer i need.
Have added a better petrol filter pre carb and regularly clean the thimbol filter on the carb too.
She is used frequently and I try to avoid the fuel sitting for long in the tank, so its fresh.
I think using E5 is a must, so its only 5% Ethanol not 10, but lining the petrol tank with a petrol liner product may be the way to go and reduce the rust caused by modern fuel. Ethanol can also attack rubber and some plastics too. but if your fuel lines have been replaced recently this may not be such an issue.
Here is Gigi parked today, and about to take the dogs for a walk and do some shopping.

My passenger side tank was leaking at bottom, separating it from the cradle was a bit awkward releasing the solder joints , but once cleaned up I could see all the pinholes, i used some fuel tank epoxy from Halfords to seal the holes initially on the outside before fibre glass matting it. Worked well
Prior to that I used some frosts fuel tank sealer on the inside after cleaning, this coated the inside well but did not seal the pin holes , it did leave the inside clean and easy to see any debris through the filler hole though , I used west system 2 part epoxy