Can you remove all the components ftom the chassis?
If so I would do so then 'pickle' it in acid for a few days. You can get "Brick acid" [hydrochloric] in 5-litre plastic bottles from your local builders-suppliers. This will strip the chassis back to bare-metal and remove any rust-pitting.
Then - thoroughly wash it, dry it, and mist-coat it with a Zinc-Chromate etch-primer [Aviation-suppliers/workshops will have this -
https://www.aircraftspruce.com/catalog/cspages/zinc.php ]
After that, the only issue is "what colour would you like it?"