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topic 11847

Restoring the gold Iridited color to die cast parts


I own a Antique motorcycle restoration Company. I have been searching for 3 years in vain to try and find a solution on how to refinish (gold Iriditing) antique carburetor bodies. The bodies are die cast zinc (pot metal). I have NOS complete carbs that truly show the correct gold Iridited finish. Some of them have more of a green rainbow effect and some have the yellow rainbow effect.

I have been told by a few companies that I could bead blast the castings in question and dip in yellow chromate solution. SO I got a gallon from a aircraft plater in my area (Dallas). He said the same thing. I got a small NOS part that the original gold was fading from and tried it. One minute, two minutes, 5 minutes,etc every time I would rinse the part in water,the gold color would wash right off ! So I figured I would leave this small part in overnight to see what effect it would bring WRONG ! When I pull the part from the solution and wiped it off with a rag,to my amazement, it FELL APART in chunks. Well that was a waste of time and a waste of a valuable part !.

I called Holley Carbs to see if they could shed some light on the subject and they were NO HELP. They just wanted $100 per body (the body of the carb would nearly fit in a package the size of a cigarette pack). Some companies tell me that I need to cadmium or zinc plate the parts before using the chromate. I sent a body off to try this. It came back gold Iridited but the surface finish of the body looked like the surface of a piece of heavily galvanized steel.? I would so much appreciate if someone would help me out of the predicament I am in. I don't want to give up on restoring these bodies,but I do want to achieve the CORRECT finish on the parts.


Charles HAllam
vintage motorcycle shop - Wills Point, Texas


Dear Charles,

We apply chromate conversion coatings to aluminum alloys rather than zinc. However I have had problems in the past coating previously coated parts via bead blasting alone. The resulting finish, if it has taken at all has been patchy at best. We blast at 30 to 40 psi using a fine glass bead media. Anything more aggressive than this causes more damage than we can accept.

If I need to coat a previously coated parts I completely strip the old coating first. This can be done in a number of ways and there are chemical formulations on the market which will do it for you, however I find the following very effective on aluminium, obviously you would have to check its suitability for your zinc castings and modify it accordingly.

1. Degrease (vapour or cold solvent)
2. Boil in water (90 degrees C +) for 15 to 30 mins, this begins to degrade the corrosion resistance of the coating. (chromate conversions on AL don't like temps in excess of 65 degrees)
3. Nitric acid clean for 5 to 10 mins (13 % ish)
4. Deoxidiser / desmutter bath
5. Ultrasonic clean in water. (if not available an old tooth brush will do the job)

As a rule I have found that the longer the components are boiled the shorter the time required in the Nitric tank. If the coating is not completely removed re treat as above but miss out the Nitric so as to avoid over etching.

Once satisfied that the components are clean try the conversion coating, it should take with ease.

Hope this helps a little, let me know.

Rich Mosley
- UK

January 3, 2008

This process works fine for aluminum parts but zinc die cast will go up in smoke in nitric acid. An old time recipe is 186 g/L sodium dichromate and 6ml sulfuric acid/L. exposure time will have to be determined. doing zinc castings is tricky because of the many variations in composition of pot metal. do not rinse vigorously just dip in a rinse tank and let dry. hope this helps.

Jan Soroka
- Dallas, Texas

June 13, 2013

Q. I'm looking for the best way to refinish Holley carburetors.

They are made of pot metal, mostly zinc. They have an aluminum baseplate as well. The zinc pot metal body was originally finished in a zinc dichromate according to Holley.

I need to find a way to strip the original finish off completely, along with any carbon and fuel deposits, and then brighten the stripped finish up chemically. Then, to apply a fresh zinc dichromate via immersion.

I believe that the original finish on these carbs was probably done using a hexavalent chromium, without zinc plating, and just relying on the zinc in the carb body to be rich enough in zinc to react with the hexavalent chromium.

I have tried using a phosphoric acid product (Alumaprep 33) to clean and activate the zinc diecast body, but it does not seem to strip what's left of the original finish, nor does it leave a bright finish. It does do a nice job of cleaning the aluminum baseplate, but it also darkens the cast aluminum. I was thinking to try muriatic acid 20:1 next.

The original finish I am looking for would be basically yellow zinc, and specifically a yellow or even tan color that likely has some oily-like residues... hues of mostly green and maybe a little red.

So in short, I need to strip, brighten, and activate cast zinc potmetal. I need to strip and brighten cast aluminum. And I need to yellow dichromate the activated zinc potmetal.

Any advice would be greatly appreciated. Google has not really been beneficial thus far!

Steve Hulett
Hobbyist - Ocala, Florida, USA

June 2013

A. Hi Steve. We have a number of threads here about restoring carburetors if you widen your search term to perhaps just "carburetor". But some knowledgable people say the only really practical approach is to have them zinc plated before re-chromating because the pot metal is just too far and variably gone to restore to a consistent look.

You are right in describing the greens and red colors as "oily-like", but oil actually has nothing to do with it. Those are diffraction colors similar to the rainbow colors of a puddle with a drop of oil on it. There is nothing red or green in the process, it's just a prism-like effect from the various thicknesses of the deposit. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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