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Old 10-28-2011, 06:07 AM   #1
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Spray painting

I am spray painting some outdoor lamps which are made from an alloy of aluminium, after bead blasting the lamps I sprayed a special alloy primer then tried to spray black gloss but dots of primer showed through the black so I tried hand painting with Valreda enamel paint and the same thing happened. has anybody got any idea why and how to overcome it, I even tried respraying with ordinary primer on top of the special one and this got the dots as well.


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Old 10-28-2011, 07:42 PM   #2
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Might you be mixing oil and acrylic sprays? Is your primer silicon based by chance?

Acrylics might stick for a time to oil primers. Oil will hardly ever stick long to acrylic primers. Silicon primers and paints, and hybrids, are a whole different breed and have not shown up yet but in anti-graffiti wall products and floor coatings for interiors as far as I know. Never worked much for wear and tear institutions so hope somebody who has pipes up.


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Old 10-29-2011, 09:38 AM   #3
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Aluminum can be very tricky to paint. A little more info on the type of paint, primer and blast media used would be helpful. Metallic type abrassives such as steel shot or grit should never be used on aluminum because it can get imbedded into the profile and start a galvanic action of dissimular metals and cause cathodic disbonding of the paint. Your blast profile my be too high for the type of paint that you are using also. If this were some type of ferrous metal, you would probably start to see pin point rusting as the paint cures and shrinks which allows the peaks of the surface profile to pierce through the dry film of the paint. Aluminum is very soft and can even be profiled with high pressure from water jetting. aluminum will oxidize immediately though a process called passivation and form its own protective barrier that should be washed with a non-hydrocarbon solvent such as laquer thinner prior to painting. An etching/wash primer will also increase bonding by mechanically and chemically biting into the metal reducing the need for a blasted profile. Most etching primers will accept a variety of top coats like acrylics, epoxies and urethanes.

Last edited by mustangmike3789; 10-30-2011 at 08:29 AM.
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