The underside of the mount has teeth. As you tighten…the teeth bite and ground. All of my mounts ground and I've never bothered to strip paint from the underside.
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This question has gone on for years. There is no better right or wrong answer. But i will say that the antenna hole saw was developed for 2 reasons. One is speed on install and the second is to that the head liner is not damaged during the install.
I have run a radio shop for a number of years and time is money. If you have to drop the head liner in some of these vehicles, it takes a while to do it. Plus you stand the chance to break or damage some of the plastic hardware that holds up the head liner. I normally open up the area above one of the rear doors to take a look to see just where the rib supports are. I use a flashlight and generally have to poke a piece of wood in to hold the small space open just enough to look in there. Normally it's just the rubber seal that you can pull away from the top of the door opening.
Using a chassis punch, you have to drill a hole large enough to get the threaded bolt through. It's real easy to slip and have the drill go in too far and tear up the head liner. Then you have to have enough opening to be able to get your arm in there to reach in enough to get to the hole you drilled. If you can reach that far, you hold the smaller section of the punch and it takes a second person to twist the bolt on the top half of the punch. So now your required to pay for two people to do the job that one install tech can do himself.
Once the bottom half of the punch is on, you can tighten it up and punch the hole. Depending on just how sharp the punch is, you may or may not have a clean hole. Chances are there is a slight upward flare on the edge of the hole going to the top of the vehicle. This may or may not seal tightly depending on how much of a flare you end up with.
So do as you please. Those that vote for the punch must have all the time in the world and someone to help them. Plus they don't care about the effort and time it takes to drop the head liner enough to reach the punch location. Then have to put it all back up with no damage.