I haave cleaned the inside with hot water and detergent as I am out of degreaser.
Next step is to pour HCL (trying to find 20 LTS of it) then I'll keep it for 2 days. Clean it again. Hopefully all the rust and residues will be eliminated. If I can't help HCl I'll either try the acid used for cleanning floors or pour vinegar but for vinegar to work it should be kept at least a week.
Then I'll take off all the paint and apply 2K clear coat only