Phosphoric Acid Treatment, Then Epoxy Primer

[Ianb]

Currently in the process of tidying and protecting the underside of my 2005 e46 M3cs, when I did the wheel wells last year I removed all the factory stonechip and blasted a pitted area of the inner arch/well, then Lechler epoxy follwed by stonechip, base and clear coat.

Because the car is inside on a lift I am unable to blast any rusty bits, it is not badly rusted just a few patches. After mechanically removing as much rust as I can, do you suggest phosphoric acid type treatment, then epoxy? DOes epoxy stick ok to the coating left by phosphoric acid, if not should I abrade it and if so does this not damage the phosphate sealing of the rust? Or should I just clean it beet I can then epoxy over whatever remains? Or are these rust neutralising primers by the various corrosion busting companies worth using? the majority of which seem to be 1k paints and look pretty much like red oxide.

Anyone used Raptor as a protective coating over epoxy rather than stonechip and paint on their cars underside, seems good but I am sure I read somewhere someone advised against it.

Any suggestions, thanks

 

[fizzy]

Epoxy mastics like Jotun 87 or Jotun 90 are designed to be used over rusted steel - as long as it isn't flaking. What they hate is nice shiny steel - doesn't stick well.
I would be inclined to skip the acid.

 

[optima21]

I just use jotun 87 and leave it at that.......literally, no stonechip or topcoat, and leave it grey so its easy to spot any damage to the coating, so far after a few years its holding up better than zinc primer and gravitex.

 

[Ianb]

What worries me, does the Hydrate 80 reduce the adhesion of the epoxy primer and is it an better than just sealing with epoxy?

 

[Ianb]

Will relook at Jotun, was going to use Lechler as I have it.

Anyone who has used both, will Jotun be better than Lechler? Are the epoxy mastics just thicker epoxy primers?

 

[optima21]

my way of thinking is that lechler is better if you want a decent surface finish, but for areas under a car where appearance doesnt matter, I'd go for Jotun, and because its thicker will give a better finish on pitted surfaces as you'll get a thinker film thickness. the lecher is designed for use on cars, the jotun is designed to be used underneath boats....

.....so on my toy boat I decided to use lechler inside as it was easier to brush on

I'll spray the outside with lechler, but was wanting to say it had been painted with proper boat paint

with the jotun you'll probably need a 2.0 to 2.5mm nozzle though.

 

[Burdekin]

Yes they do stick well to rust converters.

 

[Burdekin]

It’s not good practice to paint over rusty surfaces, removing the rust mechanically then the remaining with a rust remover is I think the way to go but I have gone the rust converter route previously and then epoxy over and it is a reasonable second best. The rust converters bond very well and the epoxy will seal the surface.

 

[Burdekin]

Will relook at Jotun, was going to use Lechler as I have it.

Anyone who has used both, will Jotun be better than Lechler? Are the epoxy mastics just thicker epoxy primers?

The mastics are more surface tolerant; they are designed to stick better when you can’t get the ideal surface preparation. Therefore good for jobs like you are doing under the car.

 

[MG David]

Ianb, not sure why being on a lift stops you blasting it. If its mess indoors you are concerned about, try one of the spot blasting guns that catch most of the media.

As I guess you know, always better to get the rust off than convert it.

 

[fizzy]

It’s not good practice to paint over rusty surfaces, removing the rust mechanically then the remaining with a rust remover is I think the way to go but I have gone the rust converter route previously and then epoxy over and it is a reasonable second best. The rust converters bond very well and the epoxy will seal the surface.

I know you have more experience with the epoxies than me but have you actually tried it?

Here is some info from Jotun on penguard which is an epoxy coating like the rest of their range.

The steel shall preferably be Rust Grade A or B (ISO 8501-1). It is technically possible to apply the coating to rust grades C and D, but it is practically challenging to ensure specified film thickness on such a rough surface, hence risk of reduced lifetime of the coating system. When steel of Rust Grade C or D is coated, the frequency of inspection and testing should be increased.

 

[Burdekin]

I know you have more experience with the epoxies than me but have you actually tried it?

Here is some info from Jotun on penguard which is an epoxy coating like the rest of their range.

The steel shall preferably be Rust Grade A or B (ISO 8501-1). It is technically possible to apply the coating to rust grades C and D, but it is practically challenging to ensure specified film thickness on such a rough surface, hence risk of reduced lifetime of the coating system. When steel of Rust Grade C or D is coated, the frequency of inspection and testing should be increased.

We use epoxies everyday and best practice is media blast to SA2.5; bonding, longevity etc is reduced with the worse surface prep. I am very surprised that Jotun are condoning painting over corrosion as they are setting themselves up for failure. We get warranty on our systems and QA needs to be spot on. No paint system will last as long over rust compared to properly prepared surface. Painting over rust is a temporary solution to hold it off for a bit longer than uncoated rust. The rust converters (and I say this from experience) over say pics A and B will also slow the rust down also but again not ideal but a lot of the time doing car stuff under the car especially we aren’t always looking for perfection. If you are looking for perfection, remove all the rust by media blasting and apply the epoxy.

 

[fizzy]

We use epoxies everyday and best practice is media blast to SA2.5; bonding, longevity etc is reduced with the worse surface prep. I am very surprised that Jotun are condoning painting over corrosion as they are setting themselves up for failure. We get warranty on our systems and QA needs to be spot on. No paint system will last as long over rust compared to properly prepared surface. Painting over rust is a temporary solution to hold it off for a bit longer than uncoated rust. The rust converters (and I say this from experience) over say pics A and B will also slow the rust down also but again not ideal but a lot of the time doing car stuff under the car especially we aren’t always looking for perfection. If you are looking for perfection, remove all the rust by media blasting and apply the epoxy.

I agree I would always blast first. But it would be interesting to see how it works over rust long term. You did some test plates didn't you a few years ago? How were they prepared?

 

[fizzy]

From my other post - no prep at all - Jotun 87 aluminium - summer grade hardener - be interesting to see how long it will last

 

[Burdekin]

I agree I would always blast first. But it would be interesting to see how it works over rust long term. You did some test plates didn't you a few years ago? How were they prepared?

They were media blasted first.

 

[Dcal]

I agree with @fizzy it

Currently in the process of tidying and protecting the underside of my 2005 e46 M3cs, when I did the wheel wells last year I removed all the factory stonechip and blasted a pitted area of the inner arch/well, then Lechler epoxy follwed by stonechip, base and clear coat.

Because the car is inside on a lift I am unable to blast any rusty bits, it is not badly rusted just a few patches. After mechanically removing as much rust as I can, do you suggest phosphoric acid type treatment, then epoxy? DOes epoxy stick ok to the coating left by phosphoric acid, if not should I abrade it and if so does this not damage the phosphate sealing of the rust? Or should I just clean it beet I can then epoxy over whatever remains? Or are these rust neutralising primers by the various corrosion busting companies worth using? the majority of which seem to be 1k paints and look pretty much like red oxide.

Anyone used Raptor as a protective coating over epoxy rather than stonechip and paint on their cars underside, seems good but I am sure I read somewhere someone advised against it.

Any suggestions, thanks

I know the mess blasting a can on a lift can make because I've done it. https://www.mig-welding.co.uk/forum...the-brink-hopefully.81348/page-7#post-1313605
It worked well and the covers contained the worst of the grit.
I also made sure the grit didn’t get into the workings of the lift.

I’m with @fizzy if you can’t blast just put the epoxy onto the best preparation you can manage.
The pre-treatments might work but the paint manufactures will not have checked compatibility so you are on your own.
Most manufactures recommend ST3 surface treatment for mechanical preparation, which takes far more work than blasting to get to the standard and is still not as good.

On the painting over rust, the manufacturers know it is second best but they will have done tests to prove its performance.
If they do recommend applying over rust the expected time to major maintenance will be reduced and most won’t recommend it in highly corrosivity categories without proper prep.

Epoxies can stick to well adhering rust but all loose rust must be removed.
The system works by excluding oxygen so if all the edges are covered to the correct thickness it can last surprisingly well.
If there is a weakness in the coating the corrosion can start again and it will fail fairly quickly.
On properly blasted steel the tendency for corrosion to creep under from the edges is greatly reduced.

 

[Ianb]

I have some Deox gel, I have tried it before but wasnt impressed, I will try it again.

Are Jotun epoxy paint available in 1l tins anywhere? Otherwise I guess its the rust buster version.

 

[Dcal]

I have some Deox gel, I have tried it before but wasnt impressed, I will try it again.

Are Jotun epoxy paint available in 1l tins anywhere? Otherwise I guess its the rust buster version.

I've only seen the 87 in 5l and 25l packs.
However 1l of EM121 is £30 +vat the Jotun is £50 + for 5l

It doesn’t go far, about 6 or 7m2 per litre per coat if brushing so you are not long getting through it.
It keeps well and has a shelf life of a couple of years but I’ve used 5 year old stuff and it’s been absolutely fine.

 

[Burdekin]

SML paints do 2.5l versions which I assume is Jotun put in their own cans.

 

[Ianb]

Thanks, I will have a look.