Today I tore out the passenger's seat and really got a good look underneath. There is quite a bit of surface corrosion. I went at it with a wire brush and carefully started peeling...
Sure enough, I've now established three 1 to 2 inch holes in the floorboard, all within 6 inches of right front seat anchor.
All of the metal that I've removed has been soft enough for me and a pliers to remove.
Now what?
the plastic-y soundproofing material is gone and I'm starting with bare metal basically.
Any help would be much appreciated!

There have been some really good threads on this... some guys told about replacing the entire floorpan... others have used all sorts of good techniques and products....

One place you should look , to see what kinds of stuff are available.. is the Eastwood Co. website.... they are huge in the restoration business.... and often package stuff in " user" friendly sizes for one project... although bulk buying is almost always cheaper....

When you do start doing stuff... remember the safety rules... including the great possibility that the paint on your car contains LEAD.... so methods which do not put your paint into particles which would get into the air are to be preferred... like liquid paint remover as compared to sandblasting... lungs/eyes/and hearing are fairly important items in the long haul....

You are going to need to make a structural analysis ..... do you just need to patch a few holes... and stop the rest of the rust.. or do you need to replace parts of the floorpan to be safe...You may need to consult your best advisors in your neighborhood...

Having gone through many similar repairs, for me the most cost effective option was fiberglass. Basically, buying the fiberglass resin and matting, cutting out most of the rotted metal, and laying fiberglass mat over the hole. After the fiberglass dries, spray a rust encapsulation style paint around the area (SEM Rust Seal) and let dry. Do this on both sides of hole. The fiberglass will be rock hard, and I did about 8 repairs like this on various parts of my floor, none of the rust has recoccured. The fiberglass seems to trap the rust from oxygen and moisture putting it in a veritable "rust tomb" IMO, fiberglass is the best compromise between actual welding in new pieces of metal, or just filling the hole with foam or something. Fiberglass IMO has the best of both worlds.

If you'd like some pictures of the repairs, I can show them to you.

I knew that there was some information out there on rust but wanted a fresh take, especially since it's my floorboards and I want to do them right the first time.
Ok, now I have several more holes. I'm trying not to look for trouble but I know that I have to be aggressive.
The 2 rear seat anchors are clear but in front and just to the rear of the the outboard rear anchor the floorpan is separating from the rocker panel...
Interesting to note that there is decent metal on top and there is still undercoating on the bottom...WITH RUSTED LAYERS IN BETWEEN!
How do tackle that? Nearby there is quite a lot of flex in the metal which is concerning from a structural standpoint.
Is there a limit to the amount of fiberglas you can use? I'm not one of these anal retentive types but then again I want to make sure I'm not doing this again next year.

The problem with fiberglass... is that once you put it on an area... some options , having to do with either welding or cutting with a torch are made incredibly unpleasant... the fumes/flames are terrible.... but if you do normal welding or to some extent brazing... you don't have those problems if you find you need to redo. or extend the repairs later....
Also, it may take some time for the fiberglass resin to stop outgassing... smelling.... so you might want to keep the windows down for a while...
Even if you wanted to fiberglass.... I would suggest you get the car structurally sound by using metal... then just use the fiberglass to protect from future damage...
There are several ways of building up fiberglass.... you will need to really research the options.. some are stronger/ lighter/thinner, etc....Remember, the strength is in the resin.. the fiber is really only there to provide a structure for the resin to adhere to....
If you don't want to weld.. or can't.... check out rivets to make it structurally sound before the fiberglass... works fine for Planes....

Yes, my husband was wondering about trying this with some rust repair we need done. The problem with this is the location... way up under the dash behind the emerg brake. We did take the car to a rust repair guy, but he inadvertantly welded over the drain hole! We could have kept a guppy in the car! We probably should have taken the car back to him to fix, but I've kind of lost confidence in him.

So, we're in the midst of trying to decide how much work the car is 'worth' (that's not our only rust spot, the bottom of hte doors need some attention), and what the best method is. Some people are of the opinion you can never 'stop' rust once it starts. But it sounds like some people have had some sucess?

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Natalie

Which method to choose depends on how long you want the repair to last. If you want a permanent solution, cut out all the rust and weld in new metal, then use good corrosion prevention techniques and chemicals and a good paint.

If you want a solution that will last but aren't worried about passing the car down to your heirs, cut out all the rust and use a good rust converter, like POR 15, then use plastic to fill the hole. Mat material is very important here, if you use quality fiberglass screen and POR, you will have a very strong repair that could outlast the rest of the car.

If you want a repair that will keep out critters and weather for couple years, go to DiscountAutoBoysZone (whatever) and get one of those body kits in a can. Oh, don't forget the latex gloves, my wife still tells stories about watching me scrape fiberglass off my hands with a steak knife.

Since we're talking passenger compartment floor here, I suggest using metal. Don't want your sweetie falling out on the highway.

Jerry

Looks like you've already recieved the whole range of options.
A few observations are in order from a guy who's done all the above at one time or another.
1) POR15 is a teriific product. It will seal the rust so that additional rusting will not occur. You can even use it with fiberglass cloth. Howevr, it is not a "converter" Converters produce a chemical reaction which changes the surface from iron oxide to iron phosphate or similar, less reactive, substance. My personal experience is thatthey are not as permanent as POR 15.

2) POR15 is expensive ( compared to other "paints")-but not as expensive as welding in new metal.

3) I do not see floor rust as a "structural" issue. These cars are full sedans with a metal roof which is stressed as a structural member. It would be more of an issue in a uni body convertible. Obviously, the floor must support the seats, but the load is not all that great. The more significant issue, for me, is the seat belt anchors. Rust in the area you describe had probably eaten away the metal below the anchors into the "B" pillar. I had a similar problem and laid up 4 layers of cloth around the seat belt anchor. I am still concerned whether it will hold in the event I need it.

4) I do not think I would try to piece-in new metal around the existing holes. If I were to weld in new metal, I would get a section of floor from a donor vehicle and weld it in from door sill to door sill, from the rear sear floor as far forward as I needed.

Well there you go--no solutions, but something to think over with your morning coffee.
Let us know how it goes.

I love a thread where only great answers are posted.....so far so good !
MS Fowler, By 'structural' I did not mean it in the 'frame twisting' sense... just that one does not want to put their foot through the floor, that the seats need to be held by the floor in an accident and of course that the seat belt anchors must be strong...

All the methods mentioned ( and that I know of ) depend on getting a good bond between the fix material and the good metal remaining ..... This usually means both chemical and physical cleanlyness...

So I usually use either phosphoric acid ( chemical etch ) or sand blasting ( with the vacuum attachment so it does not go everywhere ) ...

Sand blasting ( check out the tips which only do a few square inches at a time and hook to a vacuum ) has the advantage of giving the surface "GRIP " for stuff to hold on to....when you put paint, fiberglass etc onto metal it is a physical bond... not chemical.. so grip is important both for strength and for future moisture resistance....

The POR15 instructions are very specific about certain things like humidity and temperature and prep.... I have never heard anyone complain about POR15 not living up to its claims... in 20 years of reading about it....

If the rust involves seat anchors, I would suggest ONLY welding in new metal -- otherwise you will almost certainly have a loose seat at some time or another, and when that idiot runs the stop light you will get crushed between the seat and belt, or unnecessarily thrown about.

Other holes can be repaired "on the cheap" with sheet metal and pop rivets or fiberglass, POR 15, whatever.

You also need to find and fix the water leak that caused the problem, or it will only happen again. Like places are rusted rainwater drains, leaking windsheilds, or plugged sunroof drains. The rust starts when water pools there, and that is the lowest spot in the floorpan on the W123.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Buy a $300 Mig machine and weld everything. It's easy and will pay for itself many times over on numerous projects.

I would do as Wolfgang says. I don't know about the US, but here in the UK the annual MOT inspection would fail floor corrosion repaired with fibreglass - in fact the inspector would be very annoyed with the person attempting to put this sort of repair through, as it would damage his reputation to let through that sort of 'repair'. Especially as yours is near the seat DO NOT repair with anything other than metal. The floor is an important part of your vehicle's structure, despite it being like a tank.
the Underseal on these is of a high quality, and will remain in place even when the metal it was painted on has rusted. I tried some patch welds on mine, but kept missing further corrosion, and eventually had to grind off the plates I'd welded and replace almost the entire rear offside floor pan.

Buy a welder, learn a new valuable skill, save money, and enjoy a proper repair job rather than a quick stitch at a repair centre.

Laugh it fiberglass, but use a good resin and good sheets, and IMO its just as hard as the original, and if properly prepared is an extremely strong chemical bond. I'd rip off the entire floor before I could get the fiberglass from my rusty areas. Plus, it will never rust again. Fiberglass doesn't rust, so the little water drips going on top of your fiberglass from the windshiled gasket yer too cheap to replace will do nothing to the fiberglass. I was seriously contemplating taking out all seats and carpeting, and putting a thin layer of fiberglass over the entire floor. IMO, it is some of the greatest stuff. It has solved all of my rust issues.

Well, We have some great attitudes here... all united against RUST... my Sworn LIFETIME ENEMY....

And my heart is with Wolfgang's sentiments.... I have owned an oxy/acet rig and a TIG welder since I was 22.... and used that sort of stuff at Army autocraft shops before that....

I do not understand how anyone can live without Welding equipment....

However, I also understand that some people live in apartments, or that their car may be the only metal project which they really want to deal with....

I also have a love of Fiberglass .... ( and concrete just for the record)......

But some of the larger factors , very well put by mnorton, are not addressed in Kyle's tribute to Fiberglass..

One has to be very careful to know where the next structural member or section is if substituting something like Fiberglass.. and make provisions for what happens if the rust continues... because it will not be seen when covered by the Fiberglass... sometimes visibility and checkability are more important items when dealing with structural stuff...

Kyle, I do not know if you read my sentence where I said Fiberglass to metal is a physical bond as compared to chemical... or if that was a direct challenge... but it will be clear with little research that my description was correct... and thus my admonitions about grip and cleanlyness...

Also, if you are into Fiberglass and know about it... you should advise them on what kind of structure they should be using for this... matt, cloth, etc.....

The first thing that needs to be done is taking out all hinderances to viewing... and perhaps have one consult with someone old about the EXTENT of your rust.... then proceed on their advise,, since opinions from the forum are completely dependant on your observations... and you don't have the experience to evualate the overall condition with regards to safety... or should err on the side of carefullness in any case...

Well from my extremely limited fiberglass knowledge, I can say that layer works very well, as opposed to using one layer of thick cloth, use overlapping layers of thin cloth. Also, I find that the cloth you buy at wally world is extremely fiborous, and therefore when it dries sometimes creates jagged hard edges, while the better fiberglass mat is tightly woven together, and therfore is smooth to touch after drying. What I used on my floors was some tightly woven fiberglass my dad "requestioned" from a submarine: it was used for bulkheads and the like. The fibrous stuff is very strong, but the fibers get every where. I'd rather layer thin sheets.

Obviously, for the sake of structural integrity, it would not be adviseable to use fiberglass on seat anchors, but for holes actually in the middle of the floor, it is a cheap fix.

Also, when applying the fibeglass mat, to put fiberglass over the area you are putting the mat. You then stick the mat into place, and dab the liquid fiberglass on top. Dab and dab and dab until the cloth becomes translucent and soaked, and then let dry.

Polyster resins are most widely used in the marine industry for boats and the like, and when catalyzed through the use of methyl ethyl ketone peroxide, becomes hard. You can experiment using different amounts of catalyst, although in my opinion a long cure time is better then having it harden in 15 minutes.

When using fiberglass, use rubber gloves and if using in large amounts in an enlcosed area, a respirator or mask.

Obviously you wouldn't want an entire floor made of fiberglass, simply because it isn't very flexible, but I think small holes are definitely fiberglass territory, after the areas around them are roughed up.
Well thats my blurb.