All metals need a primer of some sort to help the paint bond with the metal. Steel is very forgiving as to which primer to use - practically anything sticks to it. If you have a BARE steel enclosure do the following:
1. Sand the surface smooth with no more than 400 grit wet-dry sandpaper.
2. Apply 2 or 3 LIGHT coats of primer and let it sit at least a day.
3. LIGHTLY wet-sand the primer with 400 grit wet-dry sandpaper - you just want to take the bumps off the surface - you know, dirt, bugs, etc.. that might have gotten in the wet prmier. To wet sand you'll need a small pail of water, a rubber or dense foam sanding block, and good wet-dry sandpaper. I like to let the sandpaper soak for 10 minutes or so before I start sanding with it. Keep the sandpaper and the workpiece wet because the water carries away the particles of primer. If your paper starts sticking to the workpiece you'll know that it's not wet enough. Wet-sanding produces a very smooth surface in a very short amount of time so be careful! Another thing to mention - don't go any smoother than 400 grit. Paint needs some 'tooth' to hold on to - if you 'buff' the metal, two thing will happen - paint won't stick to it well and it will eventually peel and you'll probably have a bunch of buffing compund all over the place and paint won't stick to that!
4. Degrease the entire enclosure with a degreaser (available at paint-supply stores) and don't touch it with bare hands again - use gloves.
5. Have a CLEAN, well-ventilated and well-lit spray area prepared. It can be outdoors. I spray water on the ground to keep the dust down.
6. Apply several LIGHT coats of paint giving adequate time to "tack-up" between coats. The order that you spray is important - leave the most exposed sides for last. In most cases this is the front face and top of the enclosure. If you want to paint the entire surface of the enclosure, inside and out, you can hang it from a bent clothes hanger - much easier to paint a hanging object than one lying on a table. Spray technique is something that comes with experience, just remember to NOT GET TOO CLOSE and use even motions.
7. Let it dry for at least an hour before touching it and at least a day before bolting parts to it.
If there is an existing painted finish on the steel and it is sticking to the steel really well (i.e. a 'factory' paint job) the last thing you want to do is sand it all off. Use it as a primer! Wet sand it with 400 grit till the gloss is gone and go to step 4.
Aluminum? If it is anodized you'll have to wet sand as much off as you can. Start with 220 grit then finish up with 400. Next you'll need an etching primer. Etching primers actually dig into the surface of the aluminum to promote adhesion. Raw aluminum - just follow the steps above substituting "etching primer" for "primer".
ok,
erix
