DIY spot rust? Need advice

[///M42 sport]

Ive been sanding down as best I could and applying por15 to the spot rusting on my undercarriage. I checked back and noticed on some areas, the rust bled through the treated areas even though I made sure the areas were dry before applying the Por15. What am I doing wrong and how should I address these other areas? I want to do the best job I can at home for right now so bringing it to the shop is out of the question. Thanks.

[packratbimmer]

If you have a PPG paint store, go there and talk to the folks at the counter. I have had amazing results with PPG DP primer.

http://us.ppgrefinish.com/getmedia/A0320685-0081-4DBE-B240-D3E0B4005CB7/09/p-196_dplf_epoxy_primer-9-13.pdf

Keep in mind that the metal needs to be cleaned properly before applying any form of paint.