This is going to sound weird,....

DON"T blast it !,.... just wire brush the surface and apply the POR 15,... from what the guy's at there booth said, It needs the rust to convert. Are you going to replace the vinyl top ?

If your going to paint the roof instead,... don't use POR-15 at all,..... just find a roof and start with the original weld and solder joints. But don't forget to tack a lot of bracing in the body first,... just like if you were going to chop a car.

good luck.