81 replies to this topic

[tiger99]

A true stainless shell would be very nice, but there are two problems. Firstly, the press tools used for mild steel will not be sufficient, and some panels would have to be made from several pieces, butt welded, because the stainless will not deform sufficiently to create the required shape. Secondly, fatigue. A stainless shell would crack in several places, and eventually fail in some possibly spectacular way. That can be designed around, but would need more than a few extra stiffeners and gussets.

The corrosion problem between mild and stainless steel is dependent on their being in electrical contact, which they will be if welded or bolted. Plus an electrolyte (impure water) has to bridge the two. So as long as the paint remained intact, there would be no corrosion. The worst problem is around the seams, a conduction path in the electrolyte which is long is not very effective, but a drop of salty water bridging a seam would be, so the seam sealer would be even more important than it normally is.

I often think about rust-free Minis, and indeed other cars too, and the best solution is galvanising. But you have to drill air vents in all the high bits and zinc drains in all the low bits, so the air can be released on entering the zinc bath, and the excess zinc drained as it is lifted out. For drainage you could leave out the rubber bungs in the floor, and tilt the shell in every direction till it was drained, but to vent air you would need to drill holes in the roof, doorsteps, and umpteen other places. And, the heat might warp the shell. But, overcome these "minor" problems, and the shell might last 50 to 100 years, if well protected in the usual way, with such things as decent paint, and stonechip underneath.

Zinc spraying by gas or plasma torch is next best, but it would take quite some time to cover the entire shell, and you would have to spray the insides of the sills and other box sections before welding them, so there would still be some vulnerable seams. But the heat input is minimal, so the shell should not distort. Has to be grit blasted first, the zinc adhesion is purely mechanical, not like hot dip galvanising, which forms an intermetallic bond.

Next best is a zinc primer. Most people here seem to use etch first, but the zinc needs to be in intimate contact with the steel to work well, so you need a zinc etch primer (impossible, the etching acid would immediately react with the zinc), grit blasting, or maybe an acid etch, rather than etch primer, to get the zinc to adhere properly.

I don't know about electroplated zinc.It should be possible, but everything would need to be squeaky clean first, even within seams, to get it to work, and would it give good coverage everywhere? And, the shell would need a bake to avoid the dreaded hydrogen embrittlement.

Right now, E coat seems to be the best practicable option. Has anyone had it done on a Mini shell?

[Jake Didsbury]

A true stainless shell would be very nice, but there are two problems. Firstly, the press tools used for mild steel will not be sufficient, and some panels would have to be made from several pieces, butt welded, because the stainless will not deform sufficiently to create the required shape. Secondly, fatigue. A stainless shell would crack in several places, and eventually fail in some possibly spectacular way. That can be designed around, but would need more than a few extra stiffeners and gussets.

The corrosion problem between mild and stainless steel is dependent on their being in electrical contact, which they will be if welded or bolted. Plus an electrolyte (impure water) has to bridge the two. So as long as the paint remained intact, there would be no corrosion. The worst problem is around the seams, a conduction path in the electrolyte which is long is not very effective, but a drop of salty water bridging a seam would be, so the seam sealer would be even more important than it normally is.

I often think about rust-free Minis, and indeed other cars too, and the best solution is galvanising. But you have to drill air vents in all the high bits and zinc drains in all the low bits, so the air can be released on entering the zinc bath, and the excess zinc drained as it is lifted out. For drainage you could leave out the rubber bungs in the floor, and tilt the shell in every direction till it was drained, but to vent air you would need to drill holes in the roof, doorsteps, and umpteen other places. And, the heat might warp the shell. But, overcome these "minor" problems, and the shell might last 50 to 100 years, if well protected in the usual way, with such things as decent paint, and stonechip underneath.

Zinc spraying by gas or plasma torch is next best, but it would take quite some time to cover the entire shell, and you would have to spray the insides of the sills and other box sections before welding them, so there would still be some vulnerable seams. But the heat input is minimal, so the shell should not distort. Has to be grit blasted first, the zinc adhesion is purely mechanical, not like hot dip galvanising, which forms an intermetallic bond.

Next best is a zinc primer. Most people here seem to use etch first, but the zinc needs to be in intimate contact with the steel to work well, so you need a zinc etch primer (impossible, the etching acid would immediately react with the zinc), grit blasting, or maybe an acid etch, rather than etch primer, to get the zinc to adhere properly.

I don't know about electroplated zinc.It should be possible, but everything would need to be squeaky clean first, even within seams, to get it to work, and would it give good coverage everywhere? And, the shell would need a bake to avoid the dreaded hydrogen embrittlement.

Right now, E coat seems to be the best practicable option. Has anyone had it done on a Mini shell?

So you cant just paint on the zinc primer onto bare metal? would it help to key the steel first?

Ive just got the subframe off mine atm and going to zince primer the new plate thast have been welded in on the heel board end under boot floor, just wondering if i can put the zinc primer directly on top of the new plates? Then im going to stone chip over the zinc primer

Thanks
Jake

[Dillius]

Stainless steel would rust. Stainless steel is made from steel, chromium and nickel. When the chromium oxidises it has a slight milky effect (look at your kitchen sink and its not as shiny as the day you bought it) this is what protects the steel from rusting.
The problem with stainless is that it is only rust proof when in the presence of oxygen and by painting it you eliminate that presence and therefore making it as corrosion proof as normal grade steel. Again if you have ever taken a kitchen sink out you'll notice a rust band around the hole the taps fit to, this is because the taps which normally have a rubber washer/seal exclude the presence of oxygen.
I hope this helps and I haven't made a t*t of myself with my very first post.

[tiger99]

Dillius,

You raise an interesting point. "Corten" steel must not be painted, it turns a red colour and protects itself from further rusting. But Corten is not a true stainless, merely corrosion resistant. Paint it at your peril. Apparently a railway company in the US did exactly that, amd ended up with holes in their carriages. And yes, stainless does sometimes rust slightly on the surface, and some grades are unsuitable for bad environments, especially where salt is involved. I don't know how well most grades would stand up to being used in a vehicle body.

But before painting, true stainless will have already developed its protective film. The difficulty is getting the paint to stick.

DeLorean used stainless as the non-structural outer skin of their rather poorly designed vehicle. I don't know if any have been successfully painted.

Jake Didsbury.

Yes, you certainly can, and should, put a good quality zinc primer on bare steel which has been abraded to give a good key. Or even better, get it gas, arc or plasma sprayed. Much better than etch primer, but only if it is going to stay in place by keying the metal first. Good for your subframe, but subframes can be hot dip galvanised, which has to be the very best thing possible. Stonechip would go over the zinc, in whatever form it is used.

If you can't key the surface adequately, etch, followed by many coats of good primer and topcoat, is best. You probably don't need more than one coat of primer if it is the epoxy type.

[Dillius]

I understood it that the protective cover the Chromium gave went soft when oxygen was excluded.

[mini-luke]

I did find this:

In certain environments, localised breakdown of the coating can lead to corrosion, which may be more severe than that experienced with un-coated surfaces and may result in high, localised, rates of attack.

http://www.bssa.org....php?article=119

Might as well make it from normal steel if that's the case.

[tiger99]

No, it is only saying that in certain conditions there may be problems. In most circumstances the correct grade of stainless will be just fine. However, it is very good information, and does point to epoxy primer being the thing. Zinc is inappropriate, and as the article points out, very bad if it gets too hot.

It would be nice if we could hot dip galvanise Mini shells as the way to end the corrosion problems. However, that is not going to happen for a number of reasons. Some, such as the risk of distortion, can possibly be overcome, but the process involves several dips, in acid for cleaning before the zinc, and in every case air must be vented from all high points and acid or zinc drained from all low points. Box sections in particular need careful venting. Whatever way you orient the shell, there are problems without practical solution, such as the need for holes in very awkward places like the middle of the roof. You can't weld up the holes afterwards without removing some zinc.

Maybe someone can solve all the problems and make it possible to use proper hot dip galvanising. Maybe a fancy jig that rotates the shell in the bath to release air, and again afterwatds to drain all the liquid traps, might be possible? Even if the process cost £1000 per shell, it would pay for itself in a few years, if you compare it to the cost of a re-shell or a professional restoration every time the rust does its mischief. And, I bet that some people would galdly pay well over £1000, considering how much work they have put into their rebuilds.

Meanwhile, chemical strip and E-coat, followed by very good primer, stonechip where appropriate, and top coat, is the best that is currently available.