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Guidelines for acceptable rack marks and other non anodized areas

Q. What percentage of an anodized part is allowed to have rack marks or air bubbles on a blind hole? What is the largest tapped hole not required to be anodized?

Hector Nevarez
Machinist - Hesperia, California, USA
November 30, 2010

simultaneous replies

A. There's no fixed answer. Every buyer must understand that some rack marks are inevitable, and every anodizer must try to minimize them. After that it's a matter of negotiation between purchaser and supplier to find a size and location both can live with.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg, South Carolina
December 2, 2010

A. There are no "guidelines" per se, although any anodizer with his salt will try to keep the rack marks to a minimum, keeping in mind that he must be able to hold the part securely, and make good electrical contact throughout the anodizing process. If you are the customer, you should be able to work with your coater on an acceptable size, and location of the rack marks.

As far as the blind hole question goes, keep in mind that the deeper and smaller diameter the hole, the more likely you will get an air bubble trapped inside. Also, lets say you were anodizing a cube, and each side had a blind hole. Odds are that at least one of the holes will potentially trap an air bubble, depending on the hole size and depth, and how the part is racked.

Your thread question is extremely vague. Usually, most tapped holes should be masked for anodizing.

Marc Green
Marc Green
anodizer - Boise, Idaho
December 2, 2010

A. Hector

The customer usually specifies (or has authority to) where rack marks are permissible, and their size & number. If not specified, the supplier uses best available methods (will vary from shop to shop) and typically is a matter of economics (what is the easiest)rather than part quality (what is the best).

If unspecified, holes, threaded or not, over 1/4" should be anodized. Should, not must.

Download a copy of the spec at

Willie Alexander
- Colorado Springs, Colorado
December 2, 2010

I am doing oil rubbed bronze plating on aluminum extruded 3 ft strips which are 1 mm in thickness and 1.3 inches in width. We are getting jigging spots that is no or very less copper at the place where the jig touches the job. The problem is not on all the pieces. The rate of rejection is around 25%. If somebody could give us an solution we would really appreciate it.


Manish Khandelwal
- Agra, India
August 25, 2012

How to avoid tool marks when anodizing

Q. I have one kind of cover needs to be anodized, but there is no holes on the cover. So on anodizing, the tool has to lock on the two sides. While it leaves the two sides obvious marks on the lock position. Could someone help to advise how to avoid it?

Jenny Cheung
- Shenzhen
June 15, 2017

A. Hi Jenny. Rack marks are an unavoidable part of anodizing. However, skillful rack design can make them smaller or in a less conspicuous spot. After that, your options are (with your customer's permission), simply apply a magic marker for cosmetic purposes (no corrosion resistance) or preferably a chromate conversion coating to offer reasonable corrosion resistance. If this is spec work you might look at MIL-A-8625 / MIL-PRF-8625 [on DLA] for the anodizing and MIL-DTL-5541 [on DLA] for the chromate conversion coating touchup.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
June 2017

A. Please share the pic of that cover ... then we can remark.

alaattin tuna
- TURKEY sakarya
August 26, 2017

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