metal fatigue

[jonathan49bat]

Over the past three months I have been renovating my E30 M3. This has been a somewhat easy affair as the car is just 18 years old and pretty well built - however this has also been a trial run in practising welding and shot blasting techniques prior to taking on a new project with a rare CSL bat.

What I have found with the M3 is that most of the metal cancer occours around seams and bracket fixings. In this respect I have removed the rubber protectant and then shot blasted any rust before applying the POR 'metal ready' and sealing paint.

My question is that where shot blasting removes the rust and still leaves the metal intact although at a redused guage I have been happy to treat. Where the shot blasting has created a hole I have cut out the area and patched in new sections of metal sheet.

I would like to know what others feel about this method? How far does one go? In an ideal world all the metal would be at the correct guage but given the shortage of panels available from Mobile tradition does one accept less guage at the sacrifice of removing complete sections like the inner wheel arch which means dismantling the entire section where the poor area is perhaps 5% of the total?

In short where does one draw the line?

Is a shell better repaired by dealing specifically with the locally poor area or should one meddle with the original integrity?

Richard

 

[chicane]

Over the past three months I have been renovating my E30 M3. This has been a somewhat easy affair as the car is just 18 years old and pretty well built - however this has also been a trial run in practising welding and shot blasting techniques prior to taking on a new project with a rare CSL bat.

What I have found with the M3 is that most of the metal cancer occours around seams and bracket fixings. In this respect I have removed the rubber protectant and then shot blasted any rust before applying the POR 'metal ready' and sealing paint.

My question is that where shot blasting removes the rust and still leaves the metal intact although at a redused guage I have been happy to treat. Where the shot blasting has created a hole I have cut out the area and patched in new sections of metal sheet.

I would like to know what others feel about this method? How far does one go? In an ideal world all the metal would be at the correct guage but given the shortage of panels available from Mobile tradition does one accept less guage at the sacrifice of removing complete sections like the inner wheel arch which means dismantling the entire section where the poor area is perhaps 5% of the total?

In short where does one draw the line?

Is a shell better repaired by dealing specifically with the locally poor area or should one meddle with the original integrity?

Richard

Any car, ANY car will have metal fatigue over time. Metal flexes, expands and contracts. You figure a 35 year old car that has been driven 100-150K over pot holes, hills, etc. even with no rust you will have microscopic stress fractures and cracks. Even if you left the car sitting in a garage for 35 years the shear 3000 lb weight of the car would cause it to sag slightly over time. To answer your question, if you remove rust (always a good thing) and then shore up that rust hole with fresh metal you are always better off than leaving it alone

 

[Mario L.]

Jonathan,

The rust spots you are finding on the E30 M3 are caused by corrosion that starts in the weld effected zone of the metal from manufacturing , plus a build up of dirt and the moisture that the dirt retains over time . .

To elaborate a bit, paint, no matter what kind you use is porous, inspected at the molecular level it is like a sponge . .

Metal in the weld effected areas oxidizes during welding, and the surface of the heated area has opened up, allowing moisture to penetrate the surface . . this oxidized metal rusts faster than bare metal.

Combine this with dirt build up and the retained moisture . . and you have what you describe . .

From my experiences . . POR encapsulates and locks out air . . an essential ingredient to stop corrosion from progressing, but as stated, cars and metal flex and you have stress cracks. Now the encapsulation is broken . .

There is only one way to stop rust, and that is to remove it entirely. Areas with rust either need to be sandbasted, glassbeaded . . then immediatly treated, I prefer self etching metal primers that can wick into tight joints and protect the bare metal, and also promote adhesion of the primers used prior to final painting. POR is too thick for this.

Brushing joints with a high quality brushable seam sealer after the application of self etching primer will seal the area from moisture with a dense rubber like flexible membrane.

It all comes down to the details . . . good luck save that E30 M3 !

 

[sesquip]

One of the great things about steel is that it has a fatigue limit - below a certain threshold of movement it will NEVER fatigue. But, of course, as has been said cars do flex, and that's why I have a crack in my E21's trunk-to-cabin bulkhead. I saw lots of the sort of rust you're talking about when I was sandblasting at the shop I used to work for. The thing to remember is that thinner metal will flex more than the original and so if you have rust-thinned areas on a structural part of the car they should be blasted and filled by welding, or replaced if the area's too big. If it's non-structural a metal-type body filler (All Metal, etc.) is generally okay. Then again, it's probably tough to find a non-structural spot on a unibody car .

By the way, none of the blends of aluminum I'm familiar with have a fatigue limit, meaning that with repeated movement of any degree they'll eventually fatigue and crack.

 

[radosc]

What you may believe or not is kind of theoretical. But I had experienced it while having a chance to drive more than 30 different e24 last 3 years. It makes SO MUCH difference if the car was rust free and well cared or is average rusty. It even makes a difference if it has been restored or not.

Original, nice cars are stiff, consistent and you feel it right away. On the road experience with sport suspension as well as with factory suspension is silk smooth ride, and fantastic cornering. But most good feeling goes from car feeling consolidated or solid (my English is not good enough to describe it).

Cars that are a bit rusty (not enough to show outside but if you look at factory welds), are much more loose. No matter how good suspension is. It feels opposite to rust free car. Inconsistent and you cannot feel the road as good. Even if you hardly notice it there is a feeling of discomfort about driving it.

On the other hand restored car are different. If it was done well, with care it will be good, but series of smaller repairs always leave trace and on the road experience is much different than if it was factory. One side welded and other rusting for example isn't going to help much.

I personally would go only with rust free car or total resto where I can have measurements made and fresh steel welded as well as factory welds corrected. And if steel is thin I wouldn't sandblast it. Instead I would blast it with baking soda or chemically remove the paint and then brush the rust. If there are holes than it makes a tough decision if you should cat it out or weld it or even fabricate a new panel. That is making a difference beetwen two restored cars. Both can look fantastic but ride quality and durability can be much different.