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Old 15th May 2006, 01:48 PM   #1
protos is offline protos  Greece
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Default How to paint al/steel chassis?

I recently tried to paint some naked alu and then a steel alloy (amplifier) chassis that was already painted white by the manufacturer .The painting was done by an auto shop worker using spray gun and some blue metallic paint left over from my car.
However in both cases this paint process chips off very easily and is practically useless.
What is the correct way to do this and what preparation/chemicals are necessary for a nice glossy finish?
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Old 15th May 2006, 01:59 PM   #2
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For the aluminium you need an etching primer. Paint will chip easily, most stuff these days is powder coated.
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Old 15th May 2006, 04:04 PM   #3
protos is offline protos  Greece
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would an epoxy primer be enough on naked aluminium?
What about painted galv steel -do you need to strip the paint off completely or just sand it a bit and apply a primer?
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Old 15th May 2006, 05:29 PM   #4
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I doubt epoxy primer is suitable. You need an acid based etching primer.

If the galvanised surface has been primed properly then just a 'key' of the original paint (800 grit wet & dry paper used wet) will be sufficient, no primer needed. If it has been painted by an amateur in the past it may need cleaning down to metal and etch primer applying as you would in the case of the aluminium enclosure.

Any new/bare metal would need keying as above.
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Old 15th May 2006, 06:54 PM   #5
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Depends on the quality of the epoxy primer used.
As a boat guy i've used a lot of epoxy primer on metal and aluminium parts. If done properly it withstands much more than the case of an amplifier. Steel and aluminium keels of sailing yachts encounter serious vibration and bending forces.
The trick is to get it totally clean, sand or grit blasting is preferred.
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Old 15th May 2006, 07:01 PM   #6
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The only way to ensure a quality painted finish on aluminum is to acid etch the surface, dye the metal and then anodize to stop further corrosion.
There are many companies that offer anodizing, however since you are probably only doing a chassis the upstart costs can be high +- $200. I use a company in San Jose CA which charges about $50 for a run.
The finish quality is great. See:
http://www.diyaudio.com/forums/attac...amp=1142180289

Let me know if you would like for inforamation
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Old 18th May 2006, 03:45 AM   #7
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It is not necessary to anodize the aluminum in this case, and I doubt if it is even worth the effort ...ever. If it was necessary, then all aluminum products everywhere would be anodized first proir to painting....and they're not.
However, if it suddenly becomes necessary for all alumnm to be treated that way , please let me know first, so I can open an Anodizing business........

A nice sanding down to bare metal...or at least down to the old primer is fine. Etch bare metal with "aluminum conditioner" (that's really just a weak acid). Wear gloves and eye protection!!!!!!!!!!!
Have your painter friend put a coat of epoxy primer on it. You should put the rest of the paint over the epoxy asap, because it adheres best, when the epoxy is still "fresh".
I have used this common bodyshop primer with perfect sucsess:
http://www.ppg.com/refinishftpsite/d...oxy_Primer.pdf

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BTW Neil...nice work, beautiful !!
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Old 18th May 2006, 04:12 AM   #8
sklimek is offline sklimek  United States
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[QUOTE]Originally posted by redrabbit
[B]
A nice sanding down to bare metal...or at least down to the old primer is fine. Etch bare metal with "aluminum conditioner" (that's really just a weak acid).
http://www.ppg.com/refinishftpsite/d...oxy_Primer.pdf

=RR=

Hi redrabbit, I did a search and found a link that described aluminum conditioner as phosphoric acid. I know that muratic acid is a low form of hydrochloric acid that they sell at hardware stores for pool cleaning on the cheap, do you believe it would work as well and what do you think the dilution ratio would be, straight, or 1-1 w/ water, or 1-10 w/ water for about 10 minutes??


Thanx - Stan
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Old 18th May 2006, 04:22 AM   #9
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HCl+Al->powder. This is much too strong of an acid even if it is diluted. Phosphoric is much weaker... more akin to a cleaning agent as opposed to a disolver of metal.
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Old 18th May 2006, 04:27 AM   #10
sklimek is offline sklimek  United States
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Does phosphoric come as a standard strength, any dilution, and what are you looking for to determine the etching time? I guess your local wholsale chemical warehouse is the place to go.

Thanx!
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