January 31 - February 6, 2011
When SSPC-SP3 (Power Tool Cleaning) is specified, how should one decide when to apply the power tool to the entire surface and when to use it just where the coating appears to be defective or rusting?
Joe Isbell of SAWS on
February 7, 2011:
1) Follow project specificatons 2) Perform adhesison testing on other surface areas that do not look bad.
Frank Rea of GPI-Southeast on
February 4, 2011:
A coatings condition assessment should be performed before writing the specification to determine if existing coatings may remain. Use a dry film thickness gage to measure the existing coating DFT. Also, make Tooke Gage observations to determine the number of coats of paint. Overcoating becomes risky when the existing coating is very thick or there are many layers. There is a limit to the number of times you can successfully overcoat existing paint.
Second, evaluate the adhesion of the existing coating in accordance with ASTM D 3359. Use Method A for a DFT of greater than 5 mils or Method B for a DFT less than five mils. It is not recommend to paint over existing coatings with an adhesion rating of less than 3A or 3B. A conservative approach is to not overcoat paint with a rating of less than 4A or 4B. Lastly, determine the type of generic resin in the existing coating by Infrared Spectroscopy. Some generic types of coating are more difficult to overcoat than others, i.e. they require more surface preparation.
Lee Edelman of Independant on
January 30, 2011:
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You would apply the power tool to the entire surface if the recoat window had expired, in order to achieve adhesion for the coating to be applied over the cleaned surface. If the existing coating has areas that are defective, power tool cleaning can be a choice of surface prep depending on the type of failure.
Power tool cleaning
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