Why do sills rust?

[CCollum]

Once the sills / frame rails are repaired from rust or replaced, how do you keep them from rusting again? How is the water getting in there in the first place?

Mine is a '64 230SL and I believe I've read somewhere that the drainage in the 230's (at least some of the earlier ones) was routed through or to the sills? I believe that was corrected in later models.

The above reason is fuzzy at best and lacking of any details obviously, so I don't want to narrow the discussion to that, but did want to throw that out there still.

Any help on understanding why the sills rust and how they can be protected from future rust would be appreciated. Thank you!!

[dirkbalter]

In my case, (66230sl) I replaced the frame rails as well. Prior to welding, I primed the insides and drilled three drain holes along the length of the rails. After welding and sealing, I coated the complete inside of the sills with what is called “Internal Frame Coating”. This product is especially for these type of applications. I replaced any rusty panels or areas on my car with new ones but the product claims that it will also encapsulate any existing rust and prevent it from further rusting as well.
To the other question, I don’t know about the design intend with respect to draining, but do know that the rails are a week spot within the body.
Hope that helps
Dirk

[DaveB]

A restorer here in Western Australia told me it's due to the way the body was coated in the factory. I don't clearly remember, but something about being dipped upside down or in some way that prevented adequate coating of the interior sill. Cavity wax sounds a good idea.

[dirkbalter]

Another area that tends to rust out, especially if the car grew up in a wet environment are the wheel arch struts. These are rusting from the inside out as well. If you haven't done already, you might want to check theses as well. They are almost impossible to change without taking the fenders off.   

[CCollum]

Thanks, guys!

Dirk, are the wheel arch struts the part that run above the wheels along the fender? I.e., the infamous part that we can't see and have to reach up to feel...to make sure it's not rusty and/or have crud sitting on top of it??

[dirkbalter]

Yes, these are the ones. If the wheel is off, you should be able to stick your head in. With a flash light, you also should be able to inspect them. I started tapping at mine with a screw driver and before long had holes left and right. The metal was paper thin in some areas. I believe most off the rusting was from the inside out on these rails.
In my opinion, what is labeled an original, rust free car isn't all that rust free a lot of times. You rails my not be as bad as mine however.
Good luck. 

[Bonnyboy]

I was inspecting a car for a friend of a friend a few years back that was all tarted up and looking pretty in a showroom.   The car was advertised as an original "Arizona car" with zero rust and he wanted it.  It was only $CAN85,000.  He had been talking to the salesman and was getting ready to arrange a flight down to Vancouver to buy it when he heard that I had one of these and may be able to go look at it. 

I stuck my hand up under the wheel well and found a big elongated rust hole on both sides, then I looked under the drivers side front rocker and there was a hole the size of a lime and could touch the drivers side floor mat, them I looked under the rear frame rail and it was patched. That was enough....   I suggested that my friend wouldn't be interested as it was definitely not rust free.   The salesman got all upset at me and explained that what I saw was typical and not enough of an issue that they couldn't call it "rust free".  Apparently the rusty metal fell off the car...

My friend was pissed when I told him as apparently one of the things he asked about was the front wheel arch struts under the front wings and they blatantly lied about it being solid.         Good luck out there - results may vary.

[Shvegel]

I had my sills replaced then the chassis was baked in an oven to remove old undercoat etc, dipped in acid to eat any remaining corrosion, neutralized and then the entire chassis was dipped in E-coat epoxy.  The nice thing about doing it this way was that all the seams where the spot welds are get primer flowed through them.  With all of this I still don't trust that the insides of the sills will be fully coated so I am going to punch a couple of 1" (25mm)holes on top of the sill under the carpet and spray cavity wax (Waxoil) in there and plug the holes with plastic covers.  I am hoping to locate the holes where I can get back to them by peeling the carpet back so when I sell the car the insides of the sills will be available for inspection.  I figure with all of the horror stories that being able to see the insides of the sills would be a big plus.

I actually used to be a collision repairman and way back when I went to a class that advocated spraying epoxy primer through a rustproofing setup in all the cavities and seams and just letting it drip off onto the floor.  If I hadn't gone the route I did this is what I would have done.  Make very sure all the loose rust and other dirt and debris is removed from the sill before using any kind of wax or primer because sealing over dirt is a great way to keep it from drying out and a pile of wet dirt sealed in a sill is a disaster.

[wwheeler]

If you can afford it, this Wurth gun does an amazing job of spraying all kinds of underbody sprays. The cavity wax Wurth has along with this gun, allows you totally coat inner panels. It is like using a spray gun with pressure and atomizes the cavity wax into a mist. It also comes with a long plastic tube with a spray nozzle that fits into 1/4" holes. So makes inner frame coating a breeze. Also does a great job of spraying undercoating. I have used it on many cars and it is still going strong.

http://www.goodspeedmotoring.com/wurth-multi-sprayer-application-gun.html

[dirkbalter]

Without questioning the quality of the Wuerth products, especially if you do these things frequently, you might also want to take a look at below links. Worked very well for me and is a bit more economic.
http://www.eastwood.com/internal-frame-coating-w-spray-nozzle-qt.html
http://www.tcpglobal.com/UPO721-2-726.html?gclid=CLvao_DB_NICFYGHfgod814MMA#.WNwT3mcm46Q
In order to remove any old coating, paint, rust .... and prepare for new coatings, my car was acid dipped and phosphor coated afterwards for rust protection, outside as well as inside the body / frame members.
Dirk

[Howard Long]

Dear Dirk,

You are correct that the strut reinforcements panels are likely to have rust.  I thought that mine on my 280SL were ok as far as from a visual inspection.  Then I read that you needed to reach into the wheel well and feel the top surface  of the strut reinforcement to determine if there was a rust problem.  Sure enough, by touch it was obvious I had rust damage on on the top panel in both wheel wells.  However, I have found that the rust did not extend back behind the removable dirt shield.

I considered taking both fenders off but that was a ways beyond my skill level.  I am now about half done and I have found that I can replace about 85% of the Strut reinforcement panels without taking off the fenders.  I am replacing all but about the aft 6 inches of each strut and I think it will look great and will be as strong or stronger than it was before. With the dirt shield back in place it will look original.

Howard
71 280SL 4-Speed

[Benz Dr.]

Well, you know the old saying.

            Yup, that there car is rust free!  You buy the car and the rust is free.

[dirkbalter]

Dr.
Yes, very true.
Howard,
Initially, I didn't want to take the fenders off either. For about a week, I was just looking at it, having a beer, looking at it, having another beer., looking at it.....
Considering my inner wheel wells as well as the fenders themselves needed work, I eventually made multiple alignment fixtures, took it all off, and ended up replacing everything. Looking back, I spend quiet a few bucks in sheet metal but I am happy I did. You are lucky it stops at the struts for you. It took a long time to get it all done.

[Howard Long]

Dirk,

I will take a few photos and let you know how it works out.

Howard

[dirkbalter]

Howard,
Sounds good.

[Benz Dr.]

It should give you an idea of how much a rust free car is really worth.