Letter 15044

Outgasing on powdercoated sandblasted steel  

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I just opened a small custom powder coating shop. We are coating a lot of used steel patio furniture. We sand blast the items and then coat them. We are having a problem with outgasing on heavily rusted areas. We are learning daily and want to put out a good finished product.

Any and all suggestions would be greatly appreciated.

David Willibey
- Bloomington, IN

David,

I assume that you have sandblasted the rust away completely. Powdercoating over rust or oxide is "never" a viable alternative, because the rust will continue to grow, and eventually will peel your topcoat. Also, if you have not completely removed all of the rust, you can have topcoat performance issues such as the ones you are having. You may want to try changing the abrasive media and/or coarsness of the media. Sandblasting can usually work well in this type of situation, however, care needs to be given to even media distibution and completeness of the area needing to be cleaned. Remember that the type of media and it's size will determine how the surface profile will look after the topcoat has been applied and cured. You may want to test different media abrasives on test parts first. Also, make sure that you have properly pretreated the metal soon after you sand blast so that it doesn't start to rust. (At this time, there is NO organic protection from the element! /s so rust comes fast). It may take a bit more impingment pressure if you are using large media. Make sure no organics are left on the part (in the valleys) before topcoat or they may well offgas during the cure cycle.

Bob Utech Brooklyn Park, MN

Editor's note:  Mr. Utech is the author of -->

A Guide to High Performance Powder Coating [link is to Amazon]

Thank-you for the response. I am wondering about the chemical reaction that creates the gas. It must be activated by the heat of the cure oven. Would a chemical bath, (pickeling ?) neutralize the corrosion? We deal with thin and very old substrates that are destroyed by the heavy forces of mechanical cleaning.

David Willibey
- Bloomington, IN

There need not be any CHEMICAL reaction. The rust is inherently porous and thus is full of air. When the component is heated to cure the coating, that air, now trapped under the coating, simply expands as any gas expands with increased temperature. Either get rid of the rust beforehand, perhaps by use of an inhibited acid pickle (which is likely covered in the archives of this site), or else impregnate the rust with some sort of resin so that the cavities in the rust are filled and thus can't store air. But the latter course could well require vacuum treatment to get good impregnation and of course the filler would have to remain inert at the coating curing temperature.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Thank you for the response. I am still confused. I am sand blasting these parts till they free of rust(I think). The parts are not white metal but they are free of rust. Some finished parts look great and others look like a sponge. There does not appear to be any rhyme or reason to what turns out good and what doesn't.

David Willibey
- Bloomington, IN

David,

We have been operating a small custom coating shop for a couple of years now. We do a lot of auto streetrod and restoration stuff. We have not had the problem that you describe but you may find this interesting. We do a lot of aluminum parts, and they outgas like crazy. What we do to prevent this is to sandblast the parts with aluminum oxide media (120 grit) then prebake the parts at 250 to 300 deg F for about 20 min to 1/2 hr. Then let cool to room temp and coat and bake the coating. The prebake seems to get rid of the gas that is trapped in the casting. This may work on your rusty steel. It's more expensive to do from a time and power point of view but it works for us. It is possible that the media that you are using is too coarse, this will close up the smaller pores and trap some rust inside that will not be clean, even though the parts look clean. Try a finer media. Do not use glass beads, doing so will really close up any minute porosity and give you nothing but grief.

Larry Catillo
- Thunder Bay, Ontario, Canada

How well does steel and aluminum pre-treated by blasting with Bicarbonate of Sodium (soda blasting) respond to powder-coating?

Thanks...

Ron Reaves
soda blasting - Memphis, TN, USA

Dave

Can you tell me if the rusting is occuring where there is a metal to metal contact. If so I would suspect that the powder coating cannot get into these areas which would cause this problem. As for the outgassing problem with casting: If you preheat the part to a higher temp then what you cure at say preheat to 400 deg then cure at 375 deg this should solve your problem.

Jim Rice
powder coating shop - Brownsburg, Indiana, USA

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