Painting problem

I don't know what I have done wrong but just painted a frame for a Landmaster rotovator. Rubbed down to bare metal one coat of Kur Rust. Two Coats of Red Oxide one coat matt black primer, then two top coats of Ransomes blue. The top coat is peeling off in sheets! just like, oh I dunno, like the skin of tomato. It is very weak one small knock makes the paint peel off.The final coat is normal brush painted enamel. Any ideas folks? Cheers Colin.