I have spent literally hours pouring over forum threads on rust, rust removal, rust repair, floor repairs, and more. I’ve learned a lot, so much that I’ve already forgotten some of it. Still, there comes a time when you just have to jump in and swim. You guys will have to bear with me as this is very new territory for me and I need some very basic advice and guidance.

First, let’s look at the problem.

This is an overview of the driver’s side floor board after peeling up the carpet, pad and coating and then stripping it down with a braid wire wheel in an angle grinder.

There are basically 6 areas that need the most attention. Along the front crease, under the pedals is area one.
This if from the top


And this is from the bottom


And here are three details from the bottom. The middle of the rusted area lies over a structural member. I’m trying to figure out what problems that poses.




Next .... along the tunnel

There are two rust-thrus along the transmission tunnel, each adjacent to a brace underneath:
Brace #1 top and bottom views



Brace #2 top and bottom views



Then there is a rust-thru at the drain hole under the gas pedal (top & bottom)



The fifth area is the rust damage around the drain hole over by the fuel lines, as seen in this top view.


And last but not least.....

And the last is a large hole in the center of the floor board that is over the structural channel underneath.



I started by using a cutting wheel on a 4-1/2” angle grinder to remove the damaged area in the center.



Only after I removed it did I realize that cutting so deep had cut into that channel underneath.





Now, for some general questions and then follow up with some specifics.

Now, for some general questions and then follow up with some specifics.
1. How far back do I need to clear off the undercoating before welding in new patches?

2. Do you all agree that the fuel lines will have to be removed before I can cut out that area and weld in a patch?

3. That center hole that I cut thru the structure, I’m guess I need to weld up the cuts first and then weld in a patch, right?

4. In the area under the pedals at the front crease, any special consideration in dealing with holes over that structural member?

5. Anyone got a good link to the basics of this type of welding/repair? Do I need to apply a weld-thru primer first? What about cold galvanizing as opposed to a primer? Just anything that you wish someone had told you before you tackled your first floor repairs.

Have you seen my thread? we should have a long conversation, lol.

2 to 3 inches on the undercoat. I became a huge fan of red oxide primer post weld, then havily coating with rustoleum truck bed coating. Also, go ahead and order some seam sealer from eastwood. Its totally amazing. Also, order some eastwood internal frame coating. There is way more to say... If it helps, you dont have much work to do. You have to pull the fuel lines though... They might be leaking without you noticing.

Ben,
JD is right, clean that undercoat back a ways, it will catch fire almost as quickly as the fuel lines (which should get moved)
Weld thru primer is great for back priming anything that will be impossible to reach later. Welding galvanized metal is very messy and does not make a strong weld unless you grind back the galvanized coating. The same really goes for any paint. Clean metal is your goal. When you grind galvanized metal you will not see sparks fly until you have gone through to bare metal so look for the sparks to know when you have got it right.
From what I have seen, you are thorough in all that you do, so don't stop now. Source a spot weld drill bit and don't be afraid to disassemble some parts to make better repairs or to repair hidden rust.
When welding thin metal, especially butt joints, a piece of copper or aluminum used as a backer will help keep things in line and act as a heat sink.
If you don't already have one, consider a welder with shielded gas over flux core wire. You will find it much cleaner and easier going.
Good luck!!

MrJD and SoFlaFiat, I have perused both your threads repeatedly, both for information and inspiration. You guys have set a standard for me to live up to. I appreciate the advice.

And now, tonight's sad, sad tale.... (also known as just another day in the life of a Fiat owner)

So tonight I started removing the undercoating using a combination of an oscillating tool with a blade, an angle grinder with a twisted wire wheel, a plastic body tool as a chisel, and a vacuum cleaner. I began by suiting up; cartridge face mask, safety goggles and ear muffs.


If you don’t have one of these mask, find your local Harbor Freight because they have them for $15 (before using my 20% coupon). It fits pretty comfortable and I know I’ll definitely be using it in my woodshop as well as any time I'm stirring rust/dust.

I suppose this is a sad story told by every Fiat owner/restorer, but the more I clean, the more rust I find. My DS floor board is beginning to resemble Swiss cheese much more than I’m comfortable with. In removing the undercoating I’m finding pinholes that weren’t readily visible topside.



I was taking before pictures and took a closer look at the patch a PO had put on under the seat.


I wasn’t real happy with what I saw. It appears that the patch was a structural consideration only with no thought about a proper patch job. Either that or the thing has just really rusted away next to it, but that doesn’t appear to be the case from what I can see with my eyes that doesn’t really show up in a picture.


And this is the view from the top



I know I have a lot more cleaning to do underneath, but there is only so much time I want to lay on my back and run power tools. At least I know what I'm in for now, maybe.

Questions:
1. How do I get into that narrow space between the rail and the door jamb? Dremel?
2. Do I consider cutting out the rail, rebuilding the area and welding the rail back in? Of small patch on the inside edge I can get to and sealing off the problem in the narrow part?
3. Regarding the pinholes in the factory channel; best method of easily dealing with them?

Ben, have no fear. My car is in almost same shape as yours. I am doing mostly butt weld. Not a fan of overlap but then again, I'm a total newby on welding. Undercoating really wants to ignite, especially oil soaked undercoating like mine! I usually am under car with wet towel while my son welds the patch. You will have to carefully work/cut around cross members as well as maybe drill out some spot welds to patch over. I would also treat inside the members before you close them up. I plan on using eastwood internal frame coating after wire brushing as much as possible. It's a rust converter. Another trick is some of those pin holes can be welded (as well as avoiding burn through) with copper plate spoons on underside. Makes for two person operation but works a charm. Cheap at Harbor Freight tools. Clean clean clean metal before welding. As for narrow spaces I just purchased an air hack saw at HF for 20 bucks. I also plan on picking up some sem copper weld through primer for those inevitable overlap joints. I had to purchase a seat rail and have to repair (weld) another. Also must section in a 3 in piece of crossmember. I have never welded or worked with metal before! Using an Eastwood 135 mig with gas shielding. Por 15 (and the prep stuff) is already purchased.
You will also have to decide what to do with drain holes/vents. Some weld them up and some leave them. I am still contemplating this
Good luck and enjoy the process!

Ben,
As much as it may seem like more work, you might find it much easier to remove the seat rail. Alternatively, a 6" wire wheel on an angle grinder should reach down into that narrow area. Hold on tight!!
Just like you, when I got to cleaning the bottom of the car I found a lot of questionable areas hidden by undercoating. Just a heads up, on my car, the whole area under the rear cross member was Swiss cheese. To the point that I was worried about the mounting points on the forward end of the long trailing arms. It's definitely worth cleaning up and checking there. The mounting points themselves are thicker metal and are usually fine but the metal around is the cheese the mice like.

I ended up removing the front half of the cross member to do repairs.



With that removed it was very easy to replace the back 8" on the passengers side floor and major patching on the drivers side.

And Ben, Happy Birthday!!

if i could not cut it from above, I cut it from below.

Also, get a good sawzall and be done with it. A few good blades (and one long one for when you cannot reach things) makes a WORLD of difference.

Keep clearing away undercoat, and you will keep finding rot. As an old boss once told me "Nothing to it but to do it."

Good luck, and if we can help, we will!

Oh, and I got lazy and started having overlapping panels near the end. Filled all the gaps left with anti-rust primer, then truck bedcoat, then seam sealer, then truck bedcoat again. Nothing is getting in there.

Can someone come up with photos and an explanation of using copper sheet or "copper plated spoons" for backing up a weld as a heat sink? What did you actually use? And how?
Trying to work up the nerve to start welding. Well, actually, still cleaning off undercoating and cutting out rust. But soon.

Ben,

Welding steel does not stick to copper. Also, copper will absorb a lot of heat to help prevent both burn through and panel warping. Spoons are simple handheld units usually making for 2 person operation. More expensive are copper plates with magnets. You would put copper on backside of patch to where you weld. I have found the spoons will fix pinholes quite well provided there is enough metal to weld, where it would otherwise just burn through without the copper.
Google copper welding spoon or go to Harbor Freight welding supplies.
Some people buy copper pipe and flatten it for same purpose. Also prices have gone up on these spoons due to the price of semi precious metals.
I have never welded before and got a mig welder with gas shielding for Christmas. It is not difficult and somewhat addictive!

Thanks, Fred, that truly helped clarify some things for me.

Spent some more time cleaning under the car, using the braided wire wheel in the angle grinder to remove the old undercoating. Not bad work, just get tired of holding my hands over my head (so to speak). Definitely glad I bought the respirator mask, safety goggles and ear muffs.
Cleaned a lot more around one of the previous patches and am seriously considering cutting it out and reworking the entire area around the outboard seat rail (driver’s side). At the upper part of the “after” pic is a rubber drain plug that seals the channel under the seat. I don’t understand this plug. However, I am thinking I can pull it and use Eastwood’s sealer up in there and replug it.
Before:

After:


I’ve got the two patches that need to be made at the tranny tunnel. Anyone got a clue on how to clean any further up (to avoid fires), short of removing the tranny? I’m guessing I’m gonna have to pull those braces anyway when I start welding, right?


I’m also thinking I’ll have to pull the wheel liner to really clean around the holes under the pedals. Sometimes I think it would be easier to just completely strip a car and start over.

Thanks for the birthday wishes, SoFlaFiat. Holy cow! When i saw that you had cut thru the cross member at the back seat, I was floored (pun intended?). I have been wondering about that area. I didn't know if I should just shoot it full of something like Eastwood's Internal Frame Coating, or find a way to truly investigate the area. Was this piece spot welded or fully welded?

Also, can someone enlighten me about seam sealer; its application, the difference between the cartridge type and the brush on, and anything else you can think of since I basically know nothing about it?

Ben,
The rear cross member is spot welded, as well as the thick brackets mounted front and back to catch the trailing arm ends. My decision to cut it down the middle and only remove the front half was based on being positive I got it back in the same place. The spot welds on the rear side are also very hard to reach. The floor under the cross member was in bad shape so I'm happy I got in there.
I always prime first, them use seam sealer under the top coat. I am not sure I am right about it but I have always been careful to allow moisture out where necessary. Sort of like leaving drain holes...