You need to prep the surface by scrubbing it really well, rinse, let it dry.

Don't mix paint types - if the primer is water-based, use water-based paints for the color coats. If the primer is solvent-based, use solvent-based paints.

You will get the best results with a roller for the first color coat, then apply the "grain" paints with a brush and the faux-painting tools

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Try this video:
http://www.youtube.com/watch?v=-rSZHQkfUFw

Here is a link that might be useful: Primer for aluminum

We painted our garage door and the BM guys suggested we use stix primer to make sure the paint would adhere to the metal first, then paint over it.

Not to be a "Danny-downer", but I'd think about this a few times....

If not done well, it looks like H=LL IMO.

On my M-I-L's condo, they had done some "wood-graining" on steel passage doors....Uggghhhh....almost hurled every time I saw them. You may hurt resale-value some too.

If you HAVE to do it, use ZAR stains.
You'll need protective topcoats too, such as a Spar Polyurethane, since the ZAR is an Oil-based stain.

NOT tryin' to be harsh here, but there's 7,000 projects I'd do b4 this one.

Faron