Hit by debris, do I need primer?
 

On my morning commute I was hit by some type of shrapnel flying across the freeway. Hit me almost dead center on the roof. Hear a loud ping, and when I got to work, sure enough, a section about 1/8" x 3/8" of missing paint at the impact zone.

I've touched up small chips on the hood/ bumper by just applying touch up paint. This is a larger area. Do I need to prime, and is there special primer needed for aluminum?

I don't know the answer to your question but don't overlook your insurance coverage. The comprehensive section of your policy would apply (missle) and a lot of people carry no deductible under that coverage. Also, a comprehensive claim under your policy would not be considered 'chargeable'. Double check your policy or check with your insurance agent.

the roof is steel. but either way, etching primer is what you would want to use first. will it make much of a difference for a touch up? probably not.