I'm working through patching up the floor pans on my 78 & noticed there was a small quarter-sized hole in the top of the cross-member that runs under the rear seat. I cut out the hole so I could patch it & noticed there is a good amount of surface rust lining the inside of this cross-member. I'm not going for show quality by any means. Just patching up the rotten metal as much as necessary to fill the basin with POR15 & get it in the condition I can drive it for another 5 yrs or so. Definitely not a full-blown restore.

Anyhow, I'm wondering what the best course of action is to handle this rust inside this cavity. It's obviously a tough spot to get at with paint. I considered cutting out a few more access holes in the top flat section so I could get some kind of spray-on rust reformer product in there to coat it but I don't want to make things worse. Here's some pictures to illustrate what I have:

If you go the Por 15 route the procedure may fail on you since the access of the xmember is limited for proper preparation, and prep is everything when it comes to Por15. Can you tell us if there are drain holes at the bottom? I had a similar issue with the inner and outer rockers and i used a long magnet to fish out loose particles and vacuumed +compressed aired the area the best i can. Then i just soaked it with rustproofing agent. I never thought about that crossmember, maybe a future procedure i may try. You can search a thread i wrote "rocker panels to rust proof or not".
If there are no other access points to this area i would leave that small cut-out accessible for future access or put a plate over it held by windshield caulk.

If you look at the first picture that shows the over-all crossmember you can see on the bottom (far left/driver's side) there is a cut-out. I'm not really sure what this was for whether for drainage or if the fuel lines were originally routed through there? On my car the fuel lines ran through two holes drilled through the floor pan just below the crossmember seam with rubber grommets, but now I'm starting to wonder if that was done after the fact? There are no holes drilled through the floor inside the crossmember that I'm aware of, but I'll get underneath & look closer.

Instead of POR15 I'm thinking about cleaning it out as best I can (I like your ideas of magnets & vacuum/compressed air) then spraying the inside as thoroughly as I can with Rustoleum Rust Reformer spray paint (http://www.rustoleum.com/product-catalo ... mer-spray/). Then welding up the access holes I cut (I'll leave that factory opening on the driver's side I mentioned above).

Does that seem reasonable?

Here's another broader view w/ the cut-out I made:

Any attention you give it is reasonable. The fuel lines routing you describe with grommets into the rear floor pan is the factory procedure.

I wonder if something like Eastwoods, Fluid Film would be better for an area you can't really get at rather than some type of rust reforming paint. I'm no expert by any means but it seems like a paint might not be so effective to an area you can't prep.

I'm going through the same thing right now on my 79. About the same amount of damage - slightly worse in some spots, slightly better in others relative to yours, but essentially the same issues in the exact same areas.

I suggest you disconnect each rear lower control arm bracket (do one at at a time and its very easy...nothing will shift since you have four other links holding the axle in place) and make sure you don't have rust holes in between the brackets and the crossmember.

My pass. side was perfect but the drivers side is kind of a mess, with 3-4 holes running horizontally beneath the top bolts and again above the bottom bolts.

I cut out the existing sheet metal that's there last weekend and will weld in 6x6" squares on each side soon. I'm going to put one on both sides just so everything remains "balanced" side to side, although adding a ply of 20 gauge sheetmetal probably wouldn't make it pull or anything like that. I'll post photos in a new thread once I get a chance.

I've read here that rust in that spot of the underside is really detrimental to the car's health. Now that I'm in there, that sheet metal doesn't really seem structural at all. The thicker bolt-on bracket is really triangulating with the crossmember backing plates.

Now if the crossmember were weak at the points around the front/top backing plates, *that* would be bad, but once I cut into the bottom sheet metal I could see they were quite clean actually.

bluespider, I'd love to see those pics. I'll have to dig into that more. My main goal here is to make the car solid for the foreseeable future. That's not a place I want to fail.

Here's the whole album of the car for any that are interested. https://goo.gl/photos/PR8TiVJeaGFyc2eU6

Here's my solution:

1. Cut one more access hole on the top of the passenger side
2. Clean out as much loose rust as I can with magnets, compressed air & vaccuum
3. Using a rag on a wire, swab the inside as thoroughly as possible with Krud Kutter rust converter
4. Spray the inside as thoroughly as possible with the Rustoleum Rust Reformer
5. Weld it back up

the guys were just talking about how vinager eats rust
you might try that
you just have to soak it for a few hours and rinse

Here is an option that worked very well for me.
I removed the front half of the cross member, cut out and replaced all rust areas in the floor, reinforcing the areas around the long trailing arm mount, cleaned and painted all the metal and reinstall the front half. It gave me great access while maintaining shape and position perfectly.

It's evident you had no choice. Very impressive surgery. Great work!

I'm glad to see this thread, as I'm facing rust in the same area on my car (see recent thread) and it is making me question whether I want to deal with it at all or let go of the car. It is encouraging to see you deal with yours!

SoFlaFiat wrote:Here is an option that worked very well for me.
I removed the front half of the cross member, cut out and replaced all rust areas in the floor, reinforcing the areas around the long trailing arm mount, cleaned and painted all the metal and reinstall the front half. It gave me great access while maintaining shape and position perfectly.

Man, that's way more involved than I really want to get. I'll crawl under the car with the wire wheel & see how things look from the bottom. I'd really prefer avoiding getting this drastic if possible. That said, having it all done right like that is appealing too... I shoulda bought a Miata