Sandblasting Changing Surface Hardness

Q. I am looking for information to relay to customers that are concerned that sandblasting chrome molly will change the hardness and molecular structure of the tubing. They will only have their tubing soda blasted. I do not feel that will provide a good adhesion for powder coating.
When using soda blasting is there special prepping that needs to be done prior to application of the powder other than a clean rinse with water? E. D.

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A. Improper grit blasting using very aggressive media can heat the part surface and cause warpage and surface softening (annealing). Your customers should be concerned about changing the surface hardness of their tubing, if the grit blasting is performed incorrectly. However, there is no way that the “molecular structure” will be affected. This is a perfect example of a little knowledge and rumor can really create a “wife’s tale.”

There is a variety of grit blast medias that will clean and prepare the surface without adversely affecting the substrate. Baking “soda,” plastic media, walnut shells, corncobs and so on are all medias that are used on very sensitive substrates like fiberglass, plastic and the like. Selecting one of these can make the blasting job much more time consuming but still fairly effective at removing mild surface contaminants. However, if you are trying to clean a heavily rusted surface, then you must bring out the heavy guns. Glass bead, silica sand, etc. can be very effective without causing surface damage and also leaves a rough surface for good powder bonding. Black beauty, steel shot, garnet, etc. are very aggressive and can cause great damage in the wrong hands.

The more aggressive the media, the faster the job will go and the easier it will be to remove heavy soils and heavy rust. However, the risk of screwing it up will also increase if you don’t know what you are doing.
My recommendation is to use the milder medias if you don’t know what you are doing and apply an iron phosphate to the part surface for better powder adhesion. If you are a very experienced blast operator, then go with a more aggressive media that will clean the surface quickly and leave surface “tooth” for good powder bonding.