by KCStudly » Wed Oct 12, 2016 9:20 am
Best prep for any coating on structural steel is "grit blast to white", meaning all rust, mill scale, contaminants, etc. have been removed and the surface has a matte finish. There are some other spec's that can be applied for removal of grease and oil prior to blasting, the specific blast media, percentage of white, cleaning off the blast media after the fact, and whatnot, but you get the point.
The big thing is that the mill scale is not necessarily a stable base for painting or powder coating. It can flake off and/or have a slick hard sheen that won't take either coating nearly as well. Yeah, some fab shops will paint right over it, but that is a low buck approach, usually to suit the cost expectations of the customer. It will look good straight out of the booth, but won't hold up as well in the field.
When I was in the lighting industry, mostly stamped sheet metal parts, we used galvanealed stock (similar to galvanized but with a finer finish), ran it thru a degreasing wash (to remove oils from the punch press operations), a phosphorous metal prep solution, drier and then onto powder coat. Mill scale was not a factor.
KC
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