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Anodizing as a base coat / primer
Q. Can I safely paint anodized aluminum parts with epoxy-polyamide paint? The base metal is 6061-T6. I need advice on which anodize to use that will properly support MIL-PRF-22750 epoxy-polyamide paint. Type I, II, III, etc... Should I specify that the anodize be sealed or left un-sealed? Any help is appreciated.
Mike BowersMilitary Contractor - Falls Church, Virginia
2006
2006
A. Mike,
Sealed or unsealed anodising? Are you talking about Chromic or sulfuric anodising?
For Chromic anodising you will find that some companies specify sealing, some don't. The Company I work for is in the latter camp. There are pros and cons to both methods.
sulfuric anodising tends to be sealed before painting.
Prior to top coating though, I would apply an epoxy primer such as
MIL-PRF-23377
[on DLA] or
MIL-PRF-85582
[from DLA]
. These will both improve adhesion of the top coat and increase corrosion resistance.
Aerospace - Yeovil, Somerset, UK
A. Safely? -- No violent reaction is expected. Use a well-ventilated paint booth and PPE to protect the painter's health.
Seriously, pre-treat and paint exactly per blueprint.
Anodize Type I or II is sealed per MIL-A-8625 / MIL-PRF-8625
[on DLA]F, type III is sealed (even dichromate sealed) if specified. Correct sealing is smut-free and may feel slick, but the proper paints will adhere. If specified, use the specified primer, e.g., MIL-PRF-23377
[on DLA] H, per specifications. Observe MIL & mfr. limits for intercoating times.
- 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.
Q. Hi,
I have a very old part whose drawing simply states "anodize" The intention of the treatment, I believe, is to work as a base coat prior to painting. Unfortunately there are no records of what kind of coating has been done previously. I am the OEM so can't go back to my customer!
I was unsure if the anodizing should be sealed or not. I felt that a non sealed coating might provide a better key for the top coat?
I was also unsure if the paint cleaning (oven at 150 °C approx.) may craze the anodising and if this may be a problem.
Are there any other considerations? Are there any standards for anodizing when used for this function?
The material is an A356 type alloy.
Metallurgist - UK
February 20, 2013
A. If water base paint leave unsealed, otherwise it makes no difference because most paint molecules are too large to get into the anodic pore. The normal porosity of 356 will surely anchor your paint of any kind. Yes, 150 °C, or any heat over 80 °C will crack the anodic coating, here a seal (hydrating the aluminum oxide) will help even before heating. Considering (1) only one part (2) old coatings, etc., why don't you just strip it down and apply an acid wash primer, then paint?
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
February 22, 2013
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