Was that 68 coats from 1 can? You might have been doing some very light dusting, too far from the body and you were moving the can too fast, and there was a breeze blowing in your spraying environment. And, if it is from 1 can, I don't think you'll see a very thick build anyway. I used 4 cans of Duplicolor clear for my project. It is a different type of substance, acrylic laquer, so it won't be an accurate comparison to your scenario. Some things you could try:
1. Shake the can for longer than the instructions suggest. Shake it every so often, after every few passes or so, but not while you're spraying of course.
2. Don't pull the can too far away from the surface you are spraying, start at about 12" and move closer as you feel comfortable. I tended to spray from 8"-12" away, starting about 2" off the sides, top or bottom, then passing over the body. Don't bring it too close either, 6" would be about the closest I sprayed. Check the manufacturers recommendation.
3. Find the right passing speed for the can you are spraying. Start a bit fast, slow down and observe how much paint is layed down as you slow down. You want it thick enough but not too thick that it runs.
Anyway, there is a sticky'd set of tips on using rattle cans in the tutorial section that covers more. Check it out.