If you can spray the metallic, why bother rolling? Believe me, once you try rolling I'll be amazed if you can resist just grabbing the spray gun and shoot it that way.

Someone earlier asked about Transtar 6071 2k roll-it primer. Yes I've been using it for the past year and it works well. It goes on real thick, the data sheet says 5mil compared to ~2 mil for a sprayed on primer. It cures hard so it's a heck of a surface to sand but it makes a great filler for sand scratches around repair areas. I only use it on deeply scratched areas and where there's a chance of bleed-through. The rest of the car I spray with Transtar 1k speed prime which is thinned with acetone.

The cracked areas I used speed prime on lifted both Rustoleum and Brightside, so be careful. If your paint is so old that it shows cracks then sanding, priming, and filling will not fix it! It needs to be sanded down to the next layer of paint and the 2k primer used to level it back up. Blocking and guide-coating did not show these problems but as soon as paint went on things went down the toilet. I've stripped and painted my trunk twice already because of this. The cracks and ghost lines are gone thanks to the 2k primer. Now I'm being more careful with the rest of the car.

You can be very frugal with the 2k roll-it by using small cups to measure the 4:1. I use graduated plastic cups that came with my mold-making supplies, they hold about 1oz. per cup. The cups that come with cough syrup work great, too. 2oz. of primer mixed with the appropriate amount of hardener is enough to roll at least an entire panel enough to block out deep sanding scratches. Try doing that with a spray gun! The down side is I paid ~$30 a quart, and that's through my friend that owns a bodyshop. The 1k speed prime was ~$50 a gallon.