Metal finishing Q&As since 1989

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Problems with cast (steel)

I am a supervisor at a coating company and have been given the task of solving the problem we are having with our cast, we shotblast the fitting then heat to 250 degrees before coating with epoxy powder, the problem were having is that after spraying there appears to be small pin holes in the coating, sometimes big pit holes, but the pit holes are there before we coat, any help would be appreciated,

Wayne O'Brien
- Durham, United Kingdom
2002

Castings are very porous. If you were to look at your casting under a microscope, you would see many tiny air holes that are formed during the casting process. When you coat your part, the holes are not filled with a one coat process. You may be experiencing outgasing as well. I would suggest trying a high fill primer coat first, then top coat over the primer.

Tim Magnuson
- Aberdeen, Washington
2002

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I am not sure if these are sandcastings or what? You also do not state what type of media abrasive you use to shot blast the parts. What size is the media and what pressure do you use? I think you might want to attempt thorough cleaning of any residual shot media or contamination "before" you heat the parts up. It is possible for this contamination to create further problems down the line once embedded inside the metal, especially if the sandblasting unit is used for other types of material such as cleaning parts with silicone on them. It only serves to reintroduce contaminates onto the part/s. When companies have trouble with outgassing or other types pinholing on any casting, its very important to make sure you eliminate ALL variables that may lead to the problem. Castings just don't seem to be as simple to paint as are other metals. The porousity that is common to many castings can make the applied powder finishes either fill in some of the crators and/or telegraph the! m. It is usually determined by the property of the powder itself. I have found that if you apply the powder when the part is in a higher heated form, it will usually such in the powder rather than offgas. The is contingent upon proper pretreatment of the part and is not the most advantageous or efficient way of powder application, but in a pinch, it may work.

Good luck.

Bob Utech
Benson, Minnesota
2002