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Old 06-11-2004, 06:53 AM
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Prepping aluminum

"Iím fixing a range Rover and the body panels are aluminum what do I do to make paint stick. This scares me!"

This was emailed to me off site and thought the answer may be of interest.

************************************************** **
You hear all kinds of reasons that paint wonít stick to aluminum such as oily or sweats but the bottom line is aluminum is really quite simple.
The real reason painters have a problem with adhesion to aluminum is if you take a nail
and make a small scratch the aluminum will start rusting inside of 15-20 minutes.
Thatís where the problem starts as you cannot see it- It will form "aluminum oxide" aluminums form of rust, and that where the problem starts.

With metal if you scratch it in 30 days you may see the starting of rust from humidly so common sense will dictate you get rid of the rust.

With this information what do you think happens when you DA the aluminum with an 80 grit sand paper? In short time the rusting starts.
If you ever go into a place that does soda or beer truck bodyís most of them will sand one panel with 80 grit and wash and spray a coat of epoxy right away, than they go to next panel. Yes I have been in paces that sand whole trailer and they are the same ones that are always having paint peal problems and not really backed up with business.

Proper way:
Wash panel with ONLY soap and water or wax and grease remover.
Sand with 80 grit on DA but only do one panel at a time.
Wash right away with wax and grease remover and shoot 1-2 coats of epoxy.
Let the epoxy set overnight before primer, body filler or paint is applied, this is overkill but safe!

There is one other alternative thats mandated in aviation (on passenger jets) and that is before the epoxy "Aladine" is applied
as its not possible to strip a 747 a panel at a time, not sure the spelling but this product is not normally used in automotive.

Last edited by BarryK; 06-11-2004 at 08:21 AM.
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Old 06-12-2004, 04:58 PM
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Just some info:

Alodine, also referred to as iridite, is a chromate conversion coating that is applied only to the surface of aluminum. It gives good corrosion protection as well as providing an excellent base for painting. (You can find more about it searching the web, many companies selling the coversion kits)

Back when I was an E3, we prepped the aircraft (lots of them were aluminum back then) and made sure we had a water break-free surface. We applied the alodine solution and let the process work until the color turned to a light gold. Then we stopped the process with water. After everything dried, a primer and top coat was applied.

Use to be hell when one of our crew didn't alodine correctly and the plane came back from the first flight with a big long stretch of paint peeled off the plane, ops.

Dutch
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Old 06-12-2004, 05:14 PM
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Use to be hell when one of our crew didn't alodine correctly and the plane came back from the first flight with a big long stretch of paint peeled off the plane, ops.

Dutch [/B][/QUOTE]
++++++++++++++++++++++++++++++++++++++++++

At least the plane came back!
Thats why the mandates are so tough for passenger jets. Its the fear of a chunk of paint getting in an engine and plane going down.
I sell a lot of epoxy in the aviation market and when it comes to doing anything on one of these planes every procedure gets signed off by an AP. These guys are always saying the reason they make so much money is one mistake and they loose it all!
Sleep on that every night!
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Old 06-13-2004, 08:07 PM
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Every place I worked sanded aluminum with 80-180 grit, then either acid washed or applyed a self etching primer and then urehane primer over that.
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