Rear trunk lid rust, How to repair rust for paint Options

[artluvr]

My 1974 914 has the typical bubbling paint on the rear trunk lid due to rust caused by the foam insulation. I have cut out a section on the left side of the lid and removed the foam to treat the rust. Should I replace the foam with something else to give rigidity to the lid? Also, should I go ahead and do the same thing to the right side of the lid, even though there are currently no pin holes there?

I am stripping the car for paint--my first experience. Thank you for any suggestions or experiences.

[Costa05]

I have been using Klean Strip Prep and Etch from Home Depot. Sold by the gallon for about $12. It is basically a rust converter that is similar to Rust Mort Converter by SEM. I do multiple applications with paint brush and scuff between coats until rust appears to be gone. Remember to neutralize following applications. Good reviews and happy so far with the results. Epoxy primer follows as a sealer to bare metal and paint prep.