Phosphoric acid ( flash rust )

[vw1]

So while waiting for my jotun epoxy to arrive I'm under my vw beetle removing as much rust as possible on the underside of the frame head.

Small area about 20inch by 15nch. I can't get car high enough for me to use my sandblaster. I have been using flap wheel and sandpaper in hard areas.

The area is not really heavy corroded so has come out well . In some small areas not so good so I got some phosphoric acid from rustbuster.co.uk .

Application is degreaser , dry then apply small amount of acid leave over night and wash off with water.

We all know what's next ? After a good wash down with water the flash rust kicks in. Rustbusters say on data sheet it's Ready for paint.

Surely this is bonkers ? I can't get passed the fact after we try removing as rust as possible we then wash the metal down with water to nutralize the solution.

So you good people that have used phosphoric acid talk to me please.

 

[awemawson]

The boat restoration folk wet sand blast steel work, allow the flash rust to form, then use 'rust converter' (which is basically phosphoric acid) to convert the flash rust to iron phosphate. Then they leave it to dry and paint directly on the black iron phosphate. The iron phosphate is chemically bonded to the steel and forms a very good 'key' for the paint.

 

[Kram]

Wash and dry directly before painting.

Otherwise skip washing, dry it and paint. Mild acid residue of citric or phosphate wont do any harm to adhesion.

My weld bench was done in citric and wiped dry, the areas have much less rust than the areas I used a flap disc. Its been outside in the rain for two years.

 

[caprinut]

Jenolite another phosphoric rust converter says implicitly not wash off with water either wipe off with a lint free cloth and allow to dry.

Bert

 

[Ross365]

This issue is why I used to use phosphoric, but rarely do so now.
Phosphoric should react with the surface to form metal phosphate, which is a protective coating. But after it's been applied, residual liquid phosphoric will probably leave the surface wet with the acid, which is damn slow to dry off.

For small components, I use to solve this problem by dipping them in isopropyl alcohol, which washes off the acid and leaves a surface which is dry to paint very quickly. That method seems to work very well and I have things I painted 30 years which still look good.

Where dipping in IPA is not possible, I'd be against using water and I have used a rag soaked in IPA to dry the surface, but I'm not so sure about that.

For the past 10 years, I clean the surface of whatever I'm working on and paint it with zinc phosphate primer asap (and follow up with a top coat made by the same company). I've been pleased with this approach and cannot recall the last time I used phosphoric.

 

[Myles1970]

This is my last effort, the before on the right and the after on the left. Stripped old paint off with a strip and clean disk, and soaked in phosphoric acid, not allowing it to dry for about 20 minutes. Then rinsed off with H2O allowing it to 'flash'. If you allow the phosphoric to dry, is leaves a powdery residue that's a pain to remove (repeat the acid treatment with a scotch pad & dry off)

Interestingly, I know of people who would strip the paint off and leave it outside to flash naturally as the resultant light rust would help with adhesion

 

[Neilj]

I've had a car bodyshell dipped in phosphoric prior to doing a lot of welding on it. It's started to flash rust in places so what I do is get a bit of 10% (approx) phosphoric on one the little kitchen pan scrub things (foam one side, green abrasive the other) and give the panel a rub over. This takes the flash rust off, then wipe it dry with a cloth. No water involved (apart from what's in the dilute phosphoric) and it leaves a phosphate coating on the metal to prevent further flash rusting.

 

[Ross365]

But the obvious question is that if the argument for using phosphoric is the formation of a protective layer, then why does it flash rust at all? I note that Jenolite (who have been in business since God was a boy) now produce their classic rust remover and also a rust converter and so do other companies. I'm unclear about why two products are on offer. Fortunately, I don't want to use either.

 

[JohnH]

After electrolysis I use POR15 Metal Prep.

I remove the object from the electrolysis bath and wash and brush under the tap until any black deposits have gone, then brush on metal prep for 15 minutes, never allowing it to dry, otherwise you get the white powdery deposits. You can see it change colour slightly when its ready. I then wash it off under the tap again then pop it into the sun to dry. I very seldom get any flash rusting. Probably didnt keep it wetted long enough.

I found the Safety Data Sheet;

They seem to mix additional Zinc Phosphate into the Phosphoric Acid solution, which probably helps the coating and stops any flash rusting.

So the question is, where can you buy Zinc Phosphate?

 

[Syv]

I'd leave water out of it, how about solvent based degreaser instead of water?

 

[vw1]

Well I have give the area under car another coat of phosphoric acid but this time only left for 10min. . Cleaned area off with distilled water and its does not flash of so quickly this gives me time to paint jotun 80 epoxy mastic on.

I also found after I cleaned acid off with distilled water if left any long than 30 mins it does show signs of flash rust. I used a red 3m scotch pad to remove flash rust, its a long way round process but this is also an option before applying jotun .

I have had email from sml paints and its been delivered today.

 

[MetalMonkey]

I just slap it on, let it dry and then either wire brush it or sand off any loose residue.
Providing it has adhered well to the steel it will provide a good base for your primer - can't see much advantage in washing it off.

It is important to dilute it with water though before applying. Using it in too much of a concentrated solution just leaves a sticky mess and doesn't improve the rust conversion ability.

 

[Burdekin]

I mechanically prep to provide a good key and then thoroughly clean with Upol water based and then solvent based degreasers. Sometimes I do a final clean with 50 50 methylated spirits / water. Rust doesn't provide a stable base to paint over however Jotamastic is surface tolerant so can stick to rust and seal it but I would avoid it if you can. Deox gel is great for removing rust in tricky areas.

Tac cloth before paint on outer panels.

 

[Dcal]

Well I have give the area under car another coat of phosphoric acid but this time only left for 10min. . Cleaned area off with distilled water and its does not flash of so quickly this gives me time to paint jotun 80 epoxy mastic on.

I also found after I cleaned acid off with distilled water if left any long than 30 mins it does show signs of flash rust. I used a red 3m scotch pad to remove flash rust, its a long way round process but this is also an option before applying jotun .

I have had email from sml paints and its been delivered today.

I think you are over thinking it.
A bit of flash rust won't bother the Jotamastic.
Jotun say it can be applied to rust grades A and B (and C and D with care) which is way beyond a bit of gingering.
Also, Jotun don't recommend any pre treatment for the steel other than removing contaminants and prepping the surface so I don't bother. I do however prep the metal as well as I can.

The primer will stick to rust far better than polished steel, the issue is when the rust isn't well adhered to the underlaying metal or is so thick it starts to delaminate.

 

[Burdekin]

I think you are over thinking it.
A bit of flash rust won't bother the Jotamastic.
Jotun say it can be applied to rust grades A and B (and C and D with care) which is way beyond a bit of gingering.
Also, Jotun don't recommend any pre treatment for the steel other than removing contaminants and prepping the surface so I don't bother. I do however prep the metal as well as I can.

The primer will stick to rust far better than polished steel, the issue is when the rust isn't well adhered to the underlaying metal or is so thick it starts to delaminate.

No they don't, that's only the rust grades of the steel before surface prep is carried out. The minimum surface prep is st2 but st2.5 is better.

VW1 have a read of the application guide on the SML website.

 

[Dcal]

No they don't, that's only the rust grades of the steel before surface prep is carried out. The minimum surface prep is st2 but st2.5 is better.

VW1 have a read of the application guide on the SML website.

This is true, but you are being just a little pedantic.
I did say the surface needed prepping, but you can start with a rusty surface.
You will never remove all the rust from rust grade A to D when cleaning to st2 (thorough hand and power tool cleaning) or even st3 (very thorough hand and power tool cleaning)
Picture below is of some rust grade D that I hand and power tool prepped last night.
The final prep was using a bristle blaster which is as good as you will get without blasting because it leaves a surface profile for the primer to stick to.

This still has rust, but it is more than good enough for an epoxy mastic primer.
Even blast cleaning to sa2.5 leaves some rust, for total removal you need to blast to sa3.

The surface tollorant bit is what tells you it can be applied over a less than perfectly prepaired surface and if that surface is steel it will certainly have some rust in it.

The application guide you refer to doesn't even mention rust after the prep has been carried out, but it does say to roughen the surface.
The reason I suspect it doesn't mention st2 or 3 ( which what is described in the text) is because there is no surface profile required in these specs which is super important.
The same document doesn't mention any chemical pre-treatments before applying the primer.

 

[Burdekin]

This is true, but you are being just a little pedantic.
I did say the surface needed prepping, but you can start with a rusty surface.
You will never remove all the rust from rust grade A to D when cleaning to st2 (thorough hand and power tool cleaning) or even st3 (very thorough hand and power tool cleaning)
Picture below is of some rust grade D that I hand and power tool prepped last night.
The final prep was using a bristle blaster which is as good as you will get without blasting because it leaves a surface profile for the primer to stick to.

View attachment 367286

This still has rust, but it is more than good enough for an epoxy mastic primer.
Even blast cleaning to sa2.5 leaves some rust, for total removal you need to blast to sa3.

The surface tollorant bit is what tells you it can be applied over a less than perfectly prepaired surface and if that surface is steel it will certainly have some rust in it.

The application guide you refer to doesn't even mention rust after the prep has been carried out, but it does say to roughen the surface.
The reason I suspect it doesn't mention st2 or 3 ( which what is described in the text) is because there is no surface profile required in these specs which is super important.
The same document doesn't mention any chemical pre-treatments before applying the primer.

I'm not. Epoxy will last significantly longer on clean media blasted steel. That is tested not hearsay or opinion. Surface prep directly correlates to how long the product will last. VW1 is doing a very good restoration, I normally let this incorrect advice go but that would be a disservice to VW1 and his restoration. St2 is the minimum recommended. Our guys at work use epoxy every day and our standard is minimum 2.5 and if it was on something more important we'd do a salt test and wash thoroughly. Rusty steel is not a good surface for any paint coating.

 

[vw1]

Qq

No they don't, that's only the rust grades of the steel before surface prep is carried out. The minimum surface prep is st2 but st2.5 is better.

VW1 have a read of the application guide on the SML website.

Not yet I'm but I will print data sheets off before I use jotun . Thanks.

 

[vw1]

I probably am over thinking it as usual. Underside off floor pan after phosphoric acid and wipe over with distilled water .

 

[Dcal]

I'm not. Epoxy will last significantly longer on clean media blasted steel. That is tested not hearsay or opinion. Surface prep directly correlates to how long the product will last. VW1 is doing a very good restoration, I normally let this incorrect advice go but that would be a disservice to VW1 and his restoration. St2 is the minimum recommended. Our guys at work use epoxy every day and our standard is minimum 2.5 and if it was on something more important we'd do a salt test and wash thoroughly. Rusty steel is not a good surface for any paint coating.

There is rust and there is rust.
It's just a matter of degree.
Short of blasting to sa3 (white metal) in a controlled atmospher and getting the paint on quickly, you are painting over rust. You might need a microscope to see it but it will be there.
Nobody outside of aerospace does that and the coatings have been developed to cope.
I'm not arguing that better prep won't extend the life of the coating system but there comes a point where the extra time and effort might not be necessary for a particular application.

Jotun (and all the other coating manafactuers) have certified coating systems using manual prep that have a life span in a C6 environment (in the splash zone) of 20 years to first major maintenance.
Blast it and it will last longer but that's still pretty good in about as hostile an environment as you can get.

If it's critical blast it, the surface profile is far more important than the composition of the surface of the metal.
If the metal is full of salts then get the salt levels down to below the recommended upper limits by for the system being used.