I have automotive clear on my bed boards and they are going on 11 years. There's some checking and a few spots where it's lifted, but 95% of the clear is still nailed to the bed boards.
The key to clear is that you have thin it down at least 50% and then brush on three to five coats, cutting between with some 200 paper. Then you can brush on full strength to start building it up and lastly go to spray for the last couple of coats. Then use auto rouges and polishing compounds to rub out the finish the same as you would on any automotive finish.
Those urethanes are used on bumper covers and they are very flexible and durable. As long as you are not staining and coating natural wood, automotive clear will work great, have a long lasting durable finish and will resist UV degradation. If you stain, then you can not use automotive clear because the stain will compromise the chemistry of the clear and it will never setup and cure properly. I saw a bed that was stained and then cleared and it was tacky and never did cure. Some food for thought.
