Any parts store also has battery terminal spray coating. It keeps the terminals sealed from the elements and will also work on a ground.
On my systems several years back, I used clear coat spray paint. It doesn't look so bad, and protects just as good as regular colored paints.
Normal primers don't seal well since most allow moisture to pass through them. There are exceptions, but most are sold in bulk form to be used on auto bodys and not in spray cans.
The ground is inside the car, but will still get corroded over time. A good ground is very important.
