a challenger shaker...neat!
If you have the proper argent paint, I think a nicely sanded (600-800 grit) surface should provide a decent base. The paint will add the texture. Spot painting it may be a problem. I tried that on my tail panel and it took a lot of time to blend it. It will look better and may actually be easier if you prep/paint the entire bubble.
I think it does not take a clear on top, but I am probably going to have a matte finish clear added to mine for protection.
But I am no body man by any stretch.