Monday, July 16, 2012

How To Paint Aluminium... And Get Great Results!

Ferrous, Non-ferrous - Easier, Harder?
Steel's great - you just can't kill it! OK, so the thin stuff might rot through, but the meatier components can be wire brushed, emery-clothed, dunked in acid, even kicked around with steel toe-caps, and still they're ready for another lick of paint. Quick dose of primer, chuck on a top coat of colour and everything's cool.

But Aluminium (or Aluminum if you're American), is a different story.

It doesn't take kindly to rough treatment. Steel wire brushes leave visible scratches, emery cloth, again, is just too harsh, and acid dissolves it before your very eyes, but I suppose you can still give it a boot with your steel toe-caps - just.
So how do we go about painting it?

As always with paintwork, it's all in the preparation.
Generally speaking, the more time spent getting the metal's surface perfect, the easier it is to paint and the finished product should be nothing short of professional.
Don't assume that a quick wire brushing is enough, the surface needs to be smooth. You need to "flat" out scratches with varying grades of wet and dry paper. Finish on a 400 grit and you'll be fine. Deep scratches can be time consuming, but if you have a flattish area to work on, use a block with the paper. It's much quicker and the results are far better. And you don't need to buy the proper rubber block seen in body shops, a piece of wood cut to the right size is fine. Wrap the wet and dry around the wood, use plenty of water to prevent the paper clogging and get that ally smooth. Wash the parts and dry thoroughly. Use spirit wipe to clean all the grease off (I use brake clean and it seems fine) and mask off any areas you need to keep paint free (threads, moving parts etc.). Now you should be ready for paint.


Why bare metal is best. 
Old paint, no matter how long it's been on there, can sometimes react with the paint you're about to apply. The result is a sudden, and bloody annoying, crazing and blistering of the new finish. You now have twice the work!
It's not impossible to paint over an old finish, but it must be flatted perfectly - no rough edges. Feather the paint's edges so you can see the primer underneath forming a line around the top coat. The more you can see, the less chance of you seeing a ridge once it's painted.
However, even after going to all this trouble, if it reacts, you need to get rid of the new paint and probably the original finish too. For best results, apply several "dust" coats first. This will help to prevent the reaction.


What primer?
Aluminium needs an Etch Primer before painting with a top coat, unless you're using a certain kind of paint eg. heatproof engine paints. These can be applied to bare metal, but usually have specific curing processes.
The etch primer uses chemicals to "roughen" the surface so that the paint sticks to the metal. This provides a long lasting finish which shouldn't flake off at the slightest touch. 
Here's a fork leg I've just painted along with the rear caliper bracket.


GSX750ES fork leg with 30-year old paint.

Paint stripped, flatted and ready to go. 

Etch primed.

Finally in satin black.
Painting tips?
If you have a flat area, like an engine case for instance, keep it elevated with a block of wood or something so that there is fresh air between it and the table/surface. This will prevent the spray rebounding back up and away from the edges you want to paint. And keep those flat components level. You'll be able to lace the paint on there and keep it "wet" without too much fear of runs. When it dries you'll have a smooth, shiny surface. 
Likewise, if you're painting a vertical surface, you need to make sure you keep it sufficiently "wet" to retain a good finish, but be wary of runs. Nothing worse than finishing a great paint job and then seeing drips appear. Gutting!


More soon, off to paint some more bits.