Hope it's not now too late to help with the project. Sorry, but I forgot to look for this thread for a few days.
I think the lines you are seeing are just bad spraying technique, put more clear on there until it gets close to running. If it does run, just wipe it off with clean paper towel and keep going (sounds bad but it works fine). When you put enough paint/clear on, the next pass will blend in and you wont see the line anymore. (assuming the overlap between one pass and the next is right)
If you are unhappy with the color and want to put more color on, then you'll need to sand the clear off of that area before adding the color.
If you the current color of the finish is ok, then just add more clear and don't worry about sanding it unless there is junk in the finish (bugs, dirt, etc.)
When applying clear over a color surface, there is no need to sand the color coat before spraying the clear unless there is junk in the finish. If the paint is metallic, don't sand it because the metallic flecks will never look right.
I wouldn't try to polish the finished job for a month or so, it takes a long time for paint to fully cure. The gloss of the clear, when applied correctly, should be more than shiny enough so you may never want to bother to buff it out.
Good Luck!