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Painting problem - Aluminum diecastings

Dear Friends,
My name Budi and my company doing Aluminium die casting product for Automotive industry.
Now, I have problem with 20.000 pieces of my product because when we paint it, the paint cannot stick to the product strongly and always peel off. People said it's because the silicon layer at the surface of the Aluminium product. my question is How to remove this silicon layer without changing the physical appearance?( the product colour)
Please, help me to solve this problem.
By the way, Thank you so much.

Regards,

Budi Sudarso
engineer - Indonesia
2006

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First of two simultaneous responses -- 2006

Dear sir,
I may help you. Tell me the percentage of Silicon in your alloy and what is the application of your product. Which type of Paint/coating you are trying to apply on it. Also what is your cleaning procedure. If I know all these things, I shall try me best to solve your problem.

Salman Aziz
piston manufacturing - Lahore, Punjab, Pakistan

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Second of two simultaneous responses -- 2006

Probably inadequate pretreatment.
Fluoride is necessary to remove silica and silicon. A deox/desmut (containing hydrofluoric acid or ammonium bifluoride), followed by chromating, may solve your problem.
This and alternative procedures are briefly described in ASTM D1730, 'Standard Practices for Preparation of Aluminum and Aluminum-Alloy Surfaces for Painting.'

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.