This stuf is what i mean

Product Description
Dinitrol Rost-off Primer RC900 forms an impenetrable barrier to oxygen and moisture. Ideal for cavities, panels and under-body work. Loose and flaking material should be removed but it is not necessary to clean down to bare metal. This Aerosol version of Dinitrol Converust RC 800 converts rust to a stable organic iron compound. Dries to a hard black film ready for fillers or topcoats. 400ml aerosol