Originally Posted by Madcow

You are correct, hardness level of the clear coat is caused by the mixing ratios of polyurethane with whatever thinner you use.

Nissan clear coat is usually pretty hard but it can vary quite a bit between the years. For example here is a black Nissan Altima I corrected a while back and it took a very heavy compound (I think I used V34 if I recall) to get just the swirls out.

Also you are right in using a DA, they are very safe. The risk of burning through the clear coat doesn't come in to play until you've polished multiple times using heavier compounds. Rotarys are definitely more challenging and dangerous.