I have to build a security privacy parking area on the side of my house. Will need 12-14 posts thru blacktop asphalt. I think I can use a Hilti hammer drill and convert to hammer chisel for the blacktop. Am not looking forward to using my digger bar & manual post hole digger for this project. Someone mentioned a post driver tool, either gas or air. Called HD rental and they do have a gas model (it only has a 3 1/4" barrel where the Postmasters are 3 1/2"...maybe they have adapters). Never used a post driver before. If anyone has used them, how do you keep things plumb and straight, as you go?