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-----Rust reappears after sand blasting
Q. Rust quickly reforms on the previously rusted areas of cast iron parts when sand blasted. We have several of these castings from a 1912 electric interurban streetcar being restored at the Seashore Trolley Museum. Twice I've blasted these to gray appearance with what I believe is aluminum oxide media only to have the rusty areas reappear shortly after. Attached pictures were taken 4 days after blasting.
I had intended to prime them with Awlgrip 545 2-part primer immediately after blasting but was unable to. Given the delay, I expected to have to do a quick pass again before painting but this rust seems excessive. Is there something I should do to further treat the casting before or after blasting? I suspect that rust or road salt has worked its way into casting below the surface but am not knowledgeable enough to pick the right next steps.
Ernie Eatonrestoration - Kennebunkport, Maine
September 29, 2025
A. Hi Ernie,
Chlorides, moisture, and acidity lead to rusting. Dryness and alkalinity deter rust.
After abrasive blasting, if you dip the parts in an alkaline solution, just a little bit of lye/caustic soda/sodium hydroxide in D.I./distilled water, towel dry them, then let the parts dry in the sun or a process oven, it should prevent any heavy rust for a reasonable period, although adding a subsequent dip in rust preventative oil is even better if the oil doesn't hurt your future plans..
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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Q. Thanks. We regularly find ourselves doing projects like this and I'd like to get it right. I'm curious why the area previously rusted generates rust again. If it is chlorides then is it possible to remove them? Maybe wash/soak the casting in hot water then reblast? Are they somehow embedded in the gray iron casting preventing the blasting from removing them? How would the lye wash affect adhesion and service life of subsequent coating with the epoxy primer? Awlgrip specs blasting (I think they use the term 'to white metal') but does not mention chemical treatments. If the lye is not compatible with the paint then would timely painting post blasting successfully prevent future rust? Sorry for all the questions. Thanks for your reply.
Ernie Eaton [returning]restoration - Kennebunkport Maine
A. Hi again.
It's not like the surface will be covered with lye, just that as the moisture dries it is alkaline rather than acidic. You can rinse/scrub the surface immediately before painting if you wish, although I don't think it's necessary.
and it is certainly worth trying one before the abrasive blasting.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Need quick confidential answers? $25
Need project assistance? $100/hr.
A. We are using a chemical called Hold*Blast [from Chlor-Rid] on our parts after blasting prior to electroplating, powder coating, and painting. We have no adhesion issues at all. Might be something you could look into sourcing for your projects. A little bit goes a long way because it's a concentrated product. We source it from our blast media supplier.
Taylor Eckert- Topeka, Kansas
October 21, 2025
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