Rust Formed Under Powder Coat - Hardest To Deal With??

[Ross365]

I've formed the opinion that rust which has got going underneath a powder coating is especially hard to deal with. I think maybe it is more prone to deep pitting, so you can do all the usual stuff to clean it off, and use you favourite paints, but the outcome is often poor. Perhaps the problem may also be related to the fact that powder coats are often applied to quite thin steel plate, which has been rolled and then rolled.....and that too may be more prone to pitting?

Over the years, my attitude to using phosphoric acid to deal with rust has blown hot and cold, but I'm definitely going to go back to it anytime I deal with failed powder coatings in future.

 

[fractured]

Any paint that does not stick like s**t to a blanket is just waiting to hold water and cause corrosion. I hate the black paint that is applied to replacement land rover chassis parts. It seems it is applied to fail quickly and hold just the right amount of moisture to cause the quickest possible corrosion and thus require the part replacing again in the shortest possible time. I would prefer un painted parts so that I can do my own prep and painting as at least I know what I am dealing with then!!

 

[Ross365]

Any paint that does not stick like s**t to a blanket is just waiting to hold water and cause corrosion.

I suspect that many of us have found that powder coatings often look OK, but then you find they are not bonded to the metal in large areas, but just hanging via the adjacent coating. When you peel the loose stuff off, the metal is seriously rusted.
Cannot comment on the Land Rover question.

 

[brightspark]

have a roof rack thats powder coated and a bullbar on an old car that has sat for 10 years outside its absolutely cracked peeling off and coroded got gear outside thats been painted in 2k for over ten years and its perfect

 

[ahebron]

Been there done that.
Had a Triumph Vitesse chassis powder coated years ago, never been on the road and when I started to rebuild the car I saw large amounts had spider rust underneath. Only option was to remove the powder coating, take the metal back to bare then rough it with an 80 grit 3m pad and zinc phosphate it before top coat from a can.
I do not recommend powder coating on cast iron/steel parts either as it is a bit porous

 

[droopsnoot]

This is exactly why I don't powder coat suspension parts. It looks lovely, until it gets a stonechip.

 

[bricol]

I've got a 22 yr old Caterham I built - when I looked at older ones, theiur powder coating was falling off, leaving very rtusty chassis.

They obviously sorted it - mine has lived outside, been driven in all weathers - one small area failed under the rear - the exposed chassis, suspension, roll bar are all still in excellent condition, way better than paint would be.

I've been lucky in that it hasn't been damaged, so not let any water under it.

Blasting has to be the way to remove the rust - and provide a good key for the finish - be it paint, or powder coat.

I have seen poorly prepared powder coated stuff rust quickly due to pin holes, or patchy coverage, not from damage. I suspect taking the "that'll be a fiver, mate" approach from a local powder coaters, rather than requiring a certain prep and finish, and paying properly for it, has a lot to do with it.

 

[Erie Fred]

Best paint I've seen yet for frames/chassis is the new-ish "Electrodip" or whatever they call it.
liquid paint, part is submerged, and electricity is applied like plating.

 

[Ross365]

Blasting has to be the way to remove the rust - and provide a good key for the finish - be it paint, or powder coat.

Blasting will remove the surface rust but besides the general surface rust you get these individual pits, very localised penetrations who's depth is much greater than the thickness of the surface rust, and possible, going through a good percentage of the thickness of the metal. Pin-hole leaks on steel tanks most likely started off as pitting corrosion.

And of course the problem is with these that you may not be even be able to see pitting after blasting off the surface rust and you very definitely won't see all of them. And rust in pits has this amazing tendency to come back to life and spread, even under a good paint.

This is why I'm thinking that for dealing with rust which started under powder coat, treating with phosphoric is the way to go. It should help convert the rust in the pits to "harmless" iron phosphate.

But then the next question is, does the phosphoric have to be washed off before painting, and if so, using what liquid .

Alternatively, do you let it dry and then brush it off? This brings us back to previous discussions on the Forum.
I think my personal view is that you either i) wipe it dry using a rag soaked in IPA or ii) let it dry and brush it off.

My reason for kicking this off again is that I just got my old Suffolk Punch mower out of the shed. It's a late model (when Bosch had control of the company) and it was all powder coated. I spent some time restoring it a couple of years ago, but the amount of rust it's now showing is horrible! Paint that has been very good on the underside of my cars has not done well on the powder-coated mower.

 

[Erie Fred]

Blasting will remove the surface rust but besides the general surface rust you get these individual pits, very localised penetrations who's depth is much greater than the thickness of the surface rust, and possible, going through a good percentage of the thickness of the metal. Pin-hole leaks on steel tanks most likely started off as pitting corrosion.

And of course the problem is with these that you may not be even be able to see pitting after blasting off the surface rust and you very definitely won't see all of them. And rust in pits has this amazing tendency to come back to life and spread, even under a good paint.

This is why I'm thinking that for dealing with rust which started under powder coat, treating with phosphoric is the way to go. It should help convert the rust in the pits to "harmless" iron phosphate.

But then the next question is, does the phosphoric have to be washed off before painting, and if so, using what liquid .

Alternatively, do you let it dry and then brush it off? This brings us back to previous discussions on the Forum.
I think my personal view is that you either i) wipe it dry using a rag soaked in IPA or ii) let it dry and brush it off.

My reason for kicking this off again is that I just got my old Suffolk Punch mower out of the shed. It's a late model (when Bosch had control of the company) and it was all powder coated. I spent some time restoring it a couple of years ago, but the amount of rust it's now showing is horrible! Paint that has been very good on the underside of my cars has not done well on the powder-coated mower.

All good ideas, and let's add electro rust removal in the mix somewhere too.
Maybe instead of Phospho ? Nothing to wash off before paint maybe.

 

[ajlelectronics]

I always Epoxy primer and polyurethane top coat over powder coat if possible

Why not do that instead and not bother with a less robust material next to the metal?

 

[ajlelectronics]

And of course the problem is with these that you may not be even be able to see pitting after blasting off the surface rust and you very definitely won't see all of them. And rust in pits has this amazing tendency to come back to life and spread, even under a good paint.

Without oxygen and water, the conversion to FE02 cannot happen, which is why epoxy primer is so often recommended.

 

[Ross365]

All good ideas, and let's add electro rust removal in the mix somewhere too.

I guess you are talking about electrolysis there? I don't have an opinion on that, but the current would have to go in to a pit who's depth to width ratio is high, and that raises some questions. In terms of dealing with rust where deep pitting is a big issue, I don't think I've ever seen any info that says method A better than Method B better than C.....

Blasting or grinding would be best if it's practicable, but so often, they are not.

 

[Erie Fred]

I guess you are talking about electrolysis there? I don't have an opinion on that, but the current would have to go in to a pit who's depth to width ratio is high, and that raises some questions. In terms of dealing with rust where deep pitting is a big issue, I don't think I've ever seen any info that says method A better than Method B better than C.....

Blasting or grinding would be best if it's practicable, but so often, they are not.

I've done allot of blasting, and 90% of the time use a very fine sand.
And I'm starting to think that I can't get it all out, no matter how much I blast, even all the way to white.
It still may leave corrosion in very small pits.

The more I read about the electrolysis method the more I like.

Along your line of thinking (which I agree with), I thought about depth ratios and some sort of blinding going on, like plating shops sometimes have to deal with.
However people doing it are saying rusted bolts are getting un-rusted, and clean even buried down in a tapped hole.