If you want a clear that's going to last and be able to handle some bumping I wouldn't use acrylic or nitro. I always use a 2 part catalyzed urethane and yes, they make it in a spray can! http://compare.ebay.com/like/1710666...Types&var=sbar
There's an extra cap on top of the existing cap that you put on a stem on the bottom, smack it on the ground, shake it up and you have a catalyzed product! I've worked with paint and refinishing for a long time and this is by far the easiest clear i've used. Just like with any clear you have to be careful not to overload your clear around your contours. Two coats of this product (wet on wet) will give you a good, long lasting clear. This product also lays down exceptionally well to the point where I've had a few cases where no buffing was needed.
If you feel like you want to add another coat after this product dries then only do one extra. People will tell you that multiple coats of clear is the right answer but that's actually what you want to avoid. Too many coats of clear and the product will become more susceptible to cracking due to trapped solvents. PM me if you have any other questions!