How do I prevent rust inside the frame rails???

[plum500]

POR-15 is incredibly picky if it is anything but rusted metal. Mind you, on rusted metal, the stuff does certainly behave like it says. On clean metal, I think many people simply ignore their recommendations for prep. I did, and it certainly doesn't stick well to un-rusted metal that has not been prepped. Degreasing with a cleaner that leaves no residue is one thing, but their metal etch is what I think most people skip -- and the reason it doesn't stick. I found it sticks really well to previously painted surfaces -- really well, tough as nails. But, if you have any part that has not been done correctly, and therefore doesn't stick -- it's a real PITA. The stuff peels right off -- and trying to work that spot, fix it, blend it to that which IS ok, is a pain.

[lexxman]

I've only used a little bit of the por 15 so far.I was told by the guy at the shop too follow the instruction to the letter or it wouldn't work correctly.He also said on bear metal I should ruff it up a little.

[green69rt]

Whenever I have a floor up for replacement...Media blast all the interior of the frame rail to remove all rust and scale.
Then heres a trick I use.  Once the new floor is drilled up for the spot welds.  On the last fit check. I'll mark the frame rail at each spot weld point.  The I use those small adhesive paper circles...The size of a dime..from any office supply joint. Pull the floor out after final fit.  Put the circles over all the marks for your spots.  Then epoxy prime the sh** out of the inside of the frame rails. Once dry...pull you circles.  Prep solve the adhesive off. 
Now you have a fully sealed frame rail with bare metal weld spots.  Quick blast of weld primer over each bare spot.  Lay in your new floor and weld away.
Promise you will never have rust in that frame rail again.

Thanks to all for the advice.  I've decided to go with the quoted process and not worry about it afterward.  I have used POR15 on most of the underside, and it does take some extra work.  The metal preparation is time consuming but the finished product does seem to hold up well, I've had a hard time getting it off the places I need to weld up.

For future reference here's what I did.
1. Coated everything with POR15 STRICTLY according to the instructions.
2. Strip off the POR15 where the pan sets on the frame(marked the pan on the underside so I will know where to drill the holes.
3. Drill the pan for the plug welds then put it back in (4th time, lots of time fitting it up.)
4. Mark through the holes onto the frame with a sharpie.
5. Bought some garage sale tags (3/4 inch circles) at Office Depot and placed them on the sharpie marks.
6. Painted over everything with another coat of por15.
7. pealed off the tags.
8. Painted the bare spots and the underside of the pan where it will attach to the frame with Eastwoods weld thru primer (3 coats).
9 Put the floor back in (5th time!!)

I'll let you know how it turns out.  Hopefully I'll start welding this weekend.  I'll post pictures.