Powder coating questions

[RickM]

By now several here have had a decent amount of experience with powder coated parts. Has anyone seen rust permeating through the coated parts?
I'm familiar with treating and coating steel using convetional products but what about when using powder? Any advice on steps to prevent rust from developing or coming back on PCed parts?

[Andras Nagy]

Hi Rick,

Powdercoating is different from "painting" only in the manner in which the pigment is applied and then baked. In painting, the pigment is suspended in a "vehicle or matrix", and when painted, the matrix dries, leaving the picment on the surface. The matrix can be water based, oil based, or any other "volitile" liquid which will evaporate. Powdercoating deposits the powder pigment via an elctrostatic charge, then the ovens bake it at about 400 degrees to "melt" the pigment, and produce the hard coating.

If there is active rust (and all rust is active) under the podwercoating, and if that rust is exposed to air and moisture, it will continue to rust, just like a painted part is likely to continue to rust.

The only way to avoid future rust is to remove all existing rust, back to "sound" metal, then protect the metal with an air-impermeable coating, such as paint or powdercoating. That's what the factory does with their paint on your car (and by the way, Porsche nopw uses water-based paint, so as not to damage the environment).

Powdercoating is not a panacea or solution to all things.

[RickM]

Thanks for the response. With non-powdercoating type coatings an etching and zinc based primer can be used to further protect the cleaned part. I don't see this as an option for powder coating....or am I wrong.

[Andras Nagy]

You are correct.

Most powdercoaters will media-blast the metal to remove EVERY trace of surface contamination (including old paint, etc) just prior to spraying the dry powder, and then immediately baking the paint.

This is more important for steel pieces than aluminum, which will start to oxidize (and hence rust) almost immediatoley upon exposing the bare metal. Even waiting over-night will allow some surface oxidation to contaminate the surface.

But once the powdercoating is on, nothing will damage the metal underneath, except scratches which expose the metal underneath.

We architects specify podwercoating on handrails, and such, on projects right on the ocean, and these pieces last 20+ years without deteriation.