Where you went wrong, is the cheapy route. Rattle can paint will not hold up to the elements regardless of all the prep work you do.

The sand blasting would have removed all the rust, your prep and paint was the problem.

Frank Cox
Cox Custom Cars

No better smell than that of fresh paint. I've been building and painting cars/trucks for 47 years. Not gonna stop now.
http://www.coxcustomcars.com/