Rick is on the right track. POR 15 is not suitable for "blind' rustproofing. It does not stick to painted or coated metal at all, or loose rust. I have used this stuff extensively on bare metal, and it is a very good primer.
You want something like Waxoyl or Rusfre.
These are soft waxy coatings that can go right over anything that might be inside those cavities, and can be applied with ordinary pneumatic 'rustproofing' spray equipment.
1986 300E 5-Speed 240k mi.