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Re: [TR] paint prep

To: John Herrera <>
Subject: Re: [TR] paint prep
Date: Mon, 25 Dec 2006 18:55:47 -0500
If you are doing an extensive paint job with down-to-the-bare-metal over 
a large area, you need to either:
1. wipe down with the appropriate solution of a product called "metal 
prep," which is a phosphoric acid-based product from 3M, as I recollect, or
2. shoot with a zirc-chrome primer. 
The "metal-prep" product is an older method, and the zirc-chrome product 
in newer.

What the  metal prep does is etch the metal and removes the almost 
invisible, ultra-thin coat of oxidation that appears instantly when in 
contact with air.  It also permits a much better bonding with the 
primer-to-metal interface, and subsequent bonding of the paint to the 

The zinc-chrome primer likewise etchs/bonds via the zinc atoms to the 
steel, and sets up an electo-potential between the steel and the zinc, 
such that the zinc pulls the free electons from the iron, thus 
preventing the oxidation of the iron.  Also, it is in effect an 
impervious coating, and must be painted with the final color within a 
specific time, or be wet-sanded with at least 400-grit or finer paper, 
before applying the color coat.  The color coat will actually bond to 
the primer if shot within the time period.

I have done both, and have never had any problems.  With both, I have 
had work scratched to the bare metal, and after months of exposure, had 
no rust appear in the scratches.

Cheers, and a very Merry Christmas (or Holidays to all the others out 

John Herrera wrote:

>> It's a common misconception that primer will protect against rust.  
>> Primer is
>> designed to be porous, so the top coat can penetrate and lock into 
>> it.  It's
>> primary purpose is to improve adhesion, it does little or nothing for 
>> rust
>> protection.  It's the top coat that locks air away from the steel, so 
>> it cannot
>> rust.
>> Randall
> Randall is correct as usual. In one of my past lives, I inspected the 
> epoxy primer on the parts used to build the A-10. I had a tool that 
> measured the primer thickness and it had to fall within a plus/minus 
> spec. I forget what the spec was, but it was REALLY thin, almost 
> transparent. I wrote up the primer for being too thick many a time, 
> but never wrote up too thin primer. Too thick and the paint wouldn't 
> stick, they said. Of course this was aluminum alloy, and may not apply 
> (pun intended) to steel.
> John H.
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