Here is a great wire brush for when you are doing the ground in your vehicle. I specifically use these wire brush that feature a centering pin because they help with keeping the brush from straying to the side scratching unwanted paint. 

Here is how I do my grounds:

Drill a 1/4" hole
Use this wire brush to remove all paint within the area of the hole
Using a 1/2" nut driver on my drill I screw down the thread cutting zinc ground lug.

Using this method of grounding is the strongest and most reliable way of grounding especially when compared to using the small self tapping screws that many shops (unfortunately for them and their customers) still use.

You can buy the thread cutting grounding bolts I use at this link:
https://www.franksaudiomotive.com/product-page/zinc-thread-cutting-ground-bolt