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edge breaking for coating

edge breaking for coating

(OP)
The [link linked page]https://www.aisc.org/DynamicTaxonomyFAQs.aspx?id=1...[/link]describes the problem perfectly, suggests a minimum radius, and then goes on to recommend not using it. "Breaking the corner" allows too much variation in interpretation to ensure sufficient coating thickness in my opinion. Anyone have some specification language they're willing to share? Application is painted steel in a corrosive atmosphere, where the rust tends to begin at the corners. Thanks.

RE: edge breaking for coating

Commonsense dictates that the required degree of radiusing depends on the thickness of the coating. Thicker coatings need more radius to avoid thinning at the corners. For thick coatings used in chemical equipment lining, we use a radius of 1/8" because our linings can be that thick or even thicker.

Sorry but I have no spec reference for you on this.

RE: edge breaking for coating

The standard for edge preparation related to vessel linings would be NACE SP0178. For structural steel, as per the link, the last sentence in the text seems fine, and stripe coating would be a normal practice. Otherwise, there is the SSPC Painting Manual - Good Painting Practice that espouses the complete avoidance of sharp edges, either by design, or by post fabrication treatments.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

RE: edge breaking for coating

(OP)
Thanks. Avoidance of sharp edges still seems subjective. A butter knife is pretty dull, but still can't hold much paint on the edge.
I find that common sense is not all that common, especially among suppliers competing in a bid.
Moltenmetal and the NACE SP0178 both use the word radius, so maybe this isn't as bad as the link above suggests? I've seen reference to a 1/8" radius or chamfer, but I'm a little unsure exactly what a 1/8" chamfer is. Which dimension does the 1/8" apply to, and will my suppliers have the same understanding without my drawing it out?

RE: edge breaking for coating

Actually, a butter knife edge will have much less problems with thinning than a sharp knife edge. This is probably the reason for the recommendation to not specify a minimum radius. A little bit of edge dulling gets you most of the way to eliminating the problem. There are a number of factors other than surface tension involved in corner thinning of coatings, and any good coating will be formulated to minimize the problem. Paint viscosity and thixotropy can be used to prevent corner thinning. Keep in mind that paints are almost always designed for low surface tension for good wetting. It is high surface tension and small radius of curvature that causes corner thinning. Also, the important radius is not that of the substrate, but the radius of the surface of the wet paint.

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