Powder Clinic: Is Blasting Enough for Preparation?

Q. I am looking into making the switch from liquid to powder coating. I have an experienced powder coater who says all my aluminum and steel parts need is to be blasted, and that should be good enough. But I am finding mixed reviews online. So is there any truth to it? Can I simply sand blast my parts and go straight into finishing them? 

A. The answer is not as simple as yes or no. Many coaters do coat after a blast-only preparation. For any indoor application this is probably a very good method of preparation with a single coat of powder because the rough surface provides a very good anchor pattern for excellent mechanical adhesion. However, it does not provide any corrosion protection, so outdoor applications will require something more robust. On steel parts for outdoor, the options are to chemically treat with a good conversion coating or add a primer layer to the blasted steel. I like the primer layer. It makes the overall coating thickness greater, greatly increases resistance to moisture penetration and enhances coverage on the edges, the most corrosion-vulnerable areas of the part. On a blasted surface that needs exceptional corrosion protection, a zinc-containing powder primer can be used. The zinc in the primer provides a sacrificial layer that offers excellent corrosion protection for the steel.

For aluminum substrates, it is also possible to blast only before coating. However, aluminum has a tight oxide layer and a conversion coating will prevent premature adhesion loss. So, what would I recommend to a start-up business? If you are going to do a batch system you can rely on blast only. Be sure to keep the media clean, blow off the dust with clean, dry compressed air and make sure you have enough thickness of coating to protect the peaks in the rough surface. If the product will be used outdoors use a primer coating under the topcoat. Partially cure the primer and then remove from the oven and apply the topcoat before co-curing the coating.

If you are going to build an automated line with an in-line washer, talk to a good chemical supplier about your options. Zirconium based products are being used successfully on both aluminum and steel and that may be the best option for you.