Good evening all. I recently purchased a small 7 1/4 gauge saddle tank loco as a long term project. I’ve removed said saddle tank and found its in good shape. However, inside has obviously suffered from rust over time, and there is some detritous inside. Once i’ve cleaned it through and removed what I can, I’d like to treat the tank but my access is very limited, two small plates and the filler hole is all I can do.
Given the tank is in good condition, I’d rather try to preserve what I have before I cut it open or replace it. My question is this, what would everyone use to treat it in the long term? I have considered rust converter but it won’t be a long term solution. Painting will be difficult given the access.
The photos below show what I am working with. Any advice will be appreciated.



