[Ampera2012]

Our mechanic mentioned signs of rust on the steel frame under our 2012 Opel Ampera.

The manual says Bitumen based undercoatings can damage the original pvc/wax protection. So I went with Teroson WX980 which explicitly says that it is compatible with pvc/wax.

I could not find any other product with that explicitly stated compatibility with pvc/wax undercoatings. Most products are bitumen/asphalt based. I couldn't find any of the products recommended by GM for sale in Europe.

I started removing rust with a drill driven crimped wire cup brush.

I found it very difficult to remove all the rust. So after removing only the loose rust on some of the more accessible areas I just sprayed on the Wx980 anyway. An additional layer after a couple of hours. I'll check the status next year and report back. Our roads are heavily salted during winter.

I also noticed that the plastic undercover on the passenger side had thick layer of gravel and debris on top/inside.

Has anyone attempted some kinf of rust protection for the undercarriage? Has anyone tried applying cavity wax?

I am a bit bewildered by researching rust protection methods, as there are so many different opinions on what products and methods to use.

 

[Wgl2020]

Horses for courses really.

Personally I would avoid underseal type products which, unless meticulously applied, can trap water and hide rust causing more issues in the future.

I have removed any plastic covers and used a wire brush to remove any loose or flakey bits, treat with a rust converter such as vactan and then overcoat with a waxy or oil based protective spray like Bilt Hamber dynax. For cavity wax Bilt Hamber s50 is easy to apply and well regarded in off road circles...

 

[Chris O’Neill]

Forget about off the shelf rust treatments,under seal will just keep moisture in and help it rust from inside out.
you need phosphoric acid.
Brush/spray into all the holes or rust,it won’t affect any good metal as it attacks the rust turning it black.
Once it’s dried (2 days) you ca just wire brush the surface stuff and primer. Lest it all dry then use stone guard on it,then new paint.
Very nasty stuff but does the job.

 

[black_amp]

I've used waxoyl products on most of my cars, seems to work well.
Phosphoric acid is goo for rust, I've always washed it off after leaving it to do it's job.

 

[Ampera2012]

Forget about off the shelf rust treatments,under seal will just keep moisture in and help it rust from inside out.
you need phosphoric acid.
Brush/spray into all the holes or rust,it won’t affect any good metal as it attacks the rust turning it black.
Once it’s dried (2 days) you ca just wire brush the surface stuff and primer. Lest it all dry then use stone guard on it,then new paint.
Very nasty stuff but does the job.

I have three concerns about phosphoric acid:

1: The chemical reaction of phosphoric acid with rust produces water, which may lead to further rust?

2. If used in excess, residual acidity may speed up corrosion if the subsequent coating is imperfect and moisture and oxygen gets in?

3. Will it damage the original coating where this is still intact?

Many sections of the steel frame on my car has an intact original coating, underneath which the metal seems to be rust free. That is why I've tried to find products that are compatible with the original coating.

I often encounter the argument that a sealing coating traps rust and moisture. But as long as only a few molecules of water are trapped, is that really a problem? The corrosion process requires a constant supply of water and or oxygen to continue, does it not? Once any trapped oxygen is consumed by oxidation underneath the coating, the corrossion process will stop. Only if the coating is compromised by abrasion, poor application, or similar, will corrosion continue again as oxgen gains contact with the metal.

The only problem with trapped rust as I see it is that the adheasion of the coating with rusted areas may be poor, which may lead to cracks and flaking faster than if bare metal is painted. Is this a misunderstanding of the chemistry involved?

 

[Chris O’Neill]

I have three concerns about phosphoric acid:

1: The chemical reaction of phosphoric acid with rust produces water, which may lead to further rust?

2. If used in excess, residual acidity may speed up corrosion if the subsequent coating is imperfect and moisture and oxygen gets in?

3. Will it damage the original coating where this is still intact?

Many sections of the steel frame on my car has an intact original coating, underneath which the metal seems to be rust free. That is why I've tried to find products that are compatible with the original coating.

I often encounter the argument that a sealing coating traps rust and moisture. But as long as only a few molecules of water are trapped, is that really a problem? The corrosion process requires a constant supply of water and or oxygen to continue, does it not? Once any trapped oxygen is consumed by oxidation underneath the coating, the corrossion process will stop. Only if the coating is compromised by abrasion, poor application, or similar, will corrosion continue again as oxgen gains contact with the metal.

The only problem with trapped rust as I see it is that the adheasion of the coating with rusted areas may be poor, which may lead to cracks and flaking faster than if bare metal is painted. Is this a misunderstanding of the chemistry involved?

I’ve noticed it leaves a white coating in places when dry,I use a wire brush to remove it.
I then use etch primer,it’s acid based.
Let its dry for 2 days,then use a stone guard over it.
Let it dry for a further 1 day.
Paint and forget.
Seems to work well that way,but you’ll never stop it completely,just slow it down.

 

[Ampera2012]

I visited the library to get to the bottom of this. The handbook on anti-corrosion painting suprisingly has little to say on the relative effectiveness of various paint systems.

It says:

- try to use the same paint as the original. Paint in good condition without rust underneath should not be removed.

- traditional (whatever that is) primers generally perform better than rust converters.

- Good adhesion is critical

Wax based coatings are not even covered by the book (which is mostly about bridges and such). PVC coatings generally perform well.

- Grease and wax should be removed before mechanical rust removal to avoid mixing grease into the metal. I assume this is for paint adhesion reasons.

In my case the original coating is PVC/wax based and I want to keep parts in good condition. In any case it is difficult to remove all of it. So a wax based coating should provide the best adhesion to the existing coating and waxy areas where the coating has worn off.

Reading some old field test results suggest that wax generally works well. See for example the evaluation below by the US military in the 1960's. More recent field research is surprisingly hard to find. The downside is that wax based coatings are soft and need to be touched up after abrasion.

https://apps.dtic.mil/dtic/tr/fulltext/u2/476493.pdf

 

[Chris O’Neill]

Try the acid approach,then etch primer.
Waxoil warmed by hot water for 10 mins will give a nice smooth coating and is runny enough to get everywhere with the spray gun and attached long pipe.
DO NOT heat the waxoil any other way as I’ve seen it explode and cover 2 guys I worked with in sticky highly flammable goo.
It caught fire,covered mikes face and hair,Jeff had his hands and face burnt very badly.
5 months off work.
Even hearing it in a bucket of hot water is dodgy as when you spray it,it’s the vapour that’s flammable.
And it stays in the air/car for a long time after.

 

[GJO]

Even hearing it in a bucket of hot water is dodgy as when you spray it,it’s the vapour that’s flammable.
And it stays in the air/car for a long time after.

What does it sound like?
To me, it doesn't sound like something that should be near high voltage electrickery . . .