Rear bumper frame won't stop bleeding, advise needed!

[PZLCruiser]

It just keeps coming back no matter what I do! First I hand sanded to bare metal and just top coated over it. Then I tried grinding the spot with a dremmel with a stone and it still fails. I do solvent wipe the area after surface prep and on the second try I used primer. The rust just seems to penetrate and blister out and oozes like a white/green powder. There is pack rust on the back side in side the pocket where the spot is. Hard to get to the inside of the frame where the issue stems from.

What should I try next?

 

[Tysano]

por15, knock the blister off with a putty knife leave the rust the por15 reacts with the rust so you just want to make the area smooth apply the por15 with a paint brush let it dry scuff it off and repaint. treat the back side also. You can buy por15 at most automotive paint suppliers.
you can also order it from ccot (this stuff kills the rust cleans up with lacquer thinner)