Powder coat

[Trigger]

I have a frame I'm going to have blasted and powder coated, but unlike previous frames I've had done this one has quite a bit of surface rust where paint has worn off over the years.

To prevent this from bubbling up under the fresh paint in a year or so I was thinking of treating it with Jenolite after blasting and maybe priming it with an anti rust primer, are both of these treatments conducive with powder coating and will it be enough? Or could I even just get away with the Jenolite and leave off the primer?

Ta.

[Drake]

If i were you, i would seek advice from you powder coating co.
They would, i suspect, get quiet a few items that maybe not in prestine condition. Maybe they could even treat it for you.

[cycleruk]

If I were you, I would seek advice from you powder coating co.
They would, I suspect, get quiet a few items that maybe not in pristine condition. Maybe they could even treat it for you.

+1
The powder is electrostatically attracted to the frame and may not stick if there is a primer etc.
Then the frame is put in an oven to cure at about 200 degC' which may cause problems with an undercoat.

[Mick F]

In order to powdercoat, the frame has to be shotblasted, therefore any primer or treatment you give it will be blasted clean away.

[al_yrpal]

At one time I was responsible for the uks biggest powder coating plant. You need to get rid of any corrosion completely. The powder naturally fills and smooths out imperfections. I would suggest a few sheets of progressivly fine wet and dry emery paper, finally used wet to completely remove any trace of corrosion. Unlike stove enamel the powder will gather more densley around edges and round them off when cured. Shot blasting is another way to ensure corroson is removed but it may not remove any deep pitting which will probably result in the imperfections showing through the finish. Thats part of the reason why you need to smooth those areas out.

Al

[pete75]

Here's a description of the process used by one company who I know do a good job. http://powder-coatings.ltd.uk/motor.php

[Brucey]

At one time I was responsible for the uks biggest powder coating plant. You need to get rid of any corrosion completely. The powder naturally fills and smooths out imperfections. I would suggest a few sheets of progressivly fine wet and dry emery paper, finally used wet to completely remove any trace of corrosion. Unlike stove enamel the powder will gather more densley around edges and round them off when cured. Shot blasting is another way to ensure corroson is removed but it may not remove any deep pitting which will probably result in the imperfections showing through the finish. Thats part of the reason why you need to smooth those areas out.

Al

I absolutely agree about removing the corrosion. However if you have pitting in a bike frame, I favour chemical means for passivating any remaining deep pits following moderate abrasion; if you try to abrade deep pits away, you might well reduce the wall thickness of the tube too much.

BTW This is the main reason why re-chroming a used bike frame is a bad idea; the prospect of hydrogen embrittlement is just the icing on an already quite nasty cake.

Al, I've worked a lot with wet paints but not so much with powdercoat. I gather that it is possible to apply a corrosion inhibiting undercoat using powdercoat systems, but I don't know how commonplace or effective this is, or indeed if there are restrictions on the chemistry (e.g. can you use a zinc-based system?) vs wet paints. I'm also aware that there is a wider choice of finishes (including metallic etc) in powdercoat these days, but not sure how good they are by comparison with others.

I'm sure I'm not the only one who would welcome any comments you might have on these topics.

cheers

[al_yrpal]

Sorry Brucey I dont know about using anything under current powder coatings. The powder coating plant I was involved with coated shop equipment, architectural metalwork, domestic appliances and vehicle parts which were usually made from zintec or similar materials. Its just like any paint system, you must be very careful about interaction between the powder and what it is coating particularly because of the high temperature which is more likely to start a reaction and the fact that there are at least two components in the powder where a reaction to fuse into a hormonogenous layer is taking place. The powder manufacturers' data sheet, or their technical advice line could probably advise of any suitable undercoating materials. I have seen some horrid mistakes occurr with paint finishes and incompatible primers. The best advice I can give is to try to remove any corrosion mechanically, the coating will smooth over imperfections so they become almost invisible.
I take your point about thinning tubes.

Al

[hamster]

I had a Dawes 531 MTB tandem in a similar condition with rusty worm trails under the paint. It was blasted and powdercoated 4 years ago. The imperfections from the trails are just visible if you know where to look (and look hard). However the coating has remained intact despite the bike spending nights outdoors when touring and bieng used off-road.

[Trigger]

In order to powdercoat, the frame has to be shotblasted, therefore any primer or treatment you give it will be blasted clean away.

I was on about treating it after the blasting, Mick, I know how it works, this will be my third bike done.

I may just risk it, the rust isn't too bad, it's just there wasn't any on the other bikes so I have nothing to go on and I'd read that blasting isn't a foolproof method of removing corrosion from the pits of the steel so there's a chance it could just come back through.

For the sake of £30 though I guess it's not worth worrying about, I'm not too bothered about the quality of finish I just want something that looks smart and is hard wearing.

[cycleruk]

Just the opposite = How does one strip a powder coated frame.?

[Brucey]

I may just risk it, the rust isn't too bad, it's just there wasn't any on the other bikes so I have nothing to go on and I'd read that blasting isn't a foolproof method of removing corrosion from the pits of the steel so there's a chance it could just come back through.

For the sake of £30 though I guess it's not worth worrying about, I'm not too bothered about the quality of finish I just want something that looks smart and is hard wearing.

if there is any pitting and it is a cheap single coat of powder coating (which it will be for £30...) then it is almost certain that the rust will come back with a vengeance, sooner or later. If corrosion starts under powder coat, it usually makes a blister in it. IMHO if the blisters are allowed to become of any size, it is possible to have severe corrosion in the underlying steel, so don't let them fester.

cheers

[Mick F]

In order to powdercoat, the frame has to be shotblasted, therefore any primer or treatment you give it will be blasted clean away.

I was on about treating it after the blasting, Mick, I know how it works, this will be my third bike done.

I may just risk it, the rust isn't too bad, it's just there wasn't any on the other bikes so I have nothing to go on and I'd read that blasting isn't a foolproof method of removing corrosion from the pits of the steel so there's a chance it could just come back through.

For the sake of £30 though I guess it's not worth worrying about, I'm not too bothered about the quality of finish I just want something that looks smart and is hard wearing.

I would expect that powdercoat is a totally sealed coating and as it goes on hot, there will be no moisture under it or to get though it, so any corrosion inside would be stopped dead. The other problem is internal, so you would have to get some frame-saver in there.

Just the opposite = How does one strip a powder coated frame.?

Shot blast it again.
Shotblasting will remove almost anything.

[Brucey]

]I would expect that powdercoat is a totally sealed coating and as it goes on hot, there will be no moisture under it or to get though it, so any corrosion inside would be stopped dead.

except that it doesn't; you might think that powder coating temperatures will drive all the moisture out of any remaining rust, but you'd be disappointed. Rust holds onto some water quite well, even at high temperatures. You need to go to well over 250C for a fair time to drive out the moisture effectively. A few minutes at 180C (as per a typical powder coat cure) is OK on perfectly clean metal but is not 100% effective on some other surfaces including any form of rust. The surface should be sealed (although it often isn't) but any trapped moisture will just cause corrosion under a sealed surface anyway.

Shotblasting will remove almost anything.

- except pitting corrosion, which is commonplace on bike frames; especially it seems, CrMo ones for some reason. A freshly shot-blasted surface can look perfectly clean, but more careful examination of the surface will usually show that although most of the rust is gone, there are tiny pits in which rust still lurks.

Hence the earlier advice re using chemical methods to passivate any pitting corrosion there might be.

cheers

[robc02]

if there is any pitting and it is a cheap single coat of powder coating (which it will be for £30...) then it is almost certain that the rust will come back with a vengeance, sooner or later.

I can vouch for this .... twice!

I'd do as much rust removal/passivation as possible before taking it for powdercoating. In both my cases the rust was around the bases of brazed-on fittings - it's possible the shotblasting didn't get in there properly.