frame rail advice

[deanj309]

Hi everyone,I removed my front floor pan,footwell pan and trunk pan.I esed a wire wheel to brush out loose dirt and a little surface rust,they are almost mint.my question is what should I use to coat the inside of the frame rails before the new pans are welded in.converter,epoxy? it.thx everyone

[NHCharger]

I brushed on a nice thick coat of SEM rust inhibitor, then sprayed on some rubberized undercoating. Tape off the top flanges of the frame rails before you spray any undercoating inside the frame rails.

[green69rt]

Clean up those insides as much as you can!!!  But in the long term don't worry about them a lot.  A forty year old car has some rust and you can keep it down with some work.  I stripped the insides of mine with a wire wheel and somehand sanding then shot it with some Metal Prep from POR15.  If you use POR15 don't even think about skipping this step.   Then wipe the excess off with a damp cloth and either brush or spray on POR15 or another rust resistant paint.   The places where you will be welding the floor, use a weld thru primer to prevent rust.  Once your are done then just forget about it because this is one of the places that you can't go back to and you will worry about nothing

Just think about it, you have a forty year old car and you need to worry about rust inside the frame fails now!!!  Do what you can and move on.

[bill440rt]

If you had the area sandblasted clean, epoxy primer would be the way to go.
If you wire wheeled them clean, then a rust-inhibitive paint such as POR-15, Eastwood's Rust Encapsulator, etc is a very good second choice.

FOLLOW THEIR PREP INSTRUCTIONS ON CAN! 
If the finish is smooth from a wire wheel, then POR-15 may not stick properly. The surface needs to be fairly rough in order for good adhesion. You could rough up the surface with some sandpaper, etc. POR-15 likes either a rough finish from rust, if the surface is clean and left rough from sandblasting that is OK, too. It needs it to "bite".
Mask of the top flanges like NH stated to coat the insides, and then paint the flanges with weld-thru primer.