How to correct a blotchy spray paint job?

You can use a fine grit sandpaper to lightly sand a little of the paint off of there are spots where the paint is a good bit thinker than the rest. If it isn’t really much thinker and all you see is that some places are darker than others I would spray another coat. With spray paint make sure you shake the can really well before you paint and shake it periodically while you are painting. Also you need to hold the can a foot or so away from the surface you are painting. It takes a few coats to get a good even coverage. Make sure you wait a little while between coats for the paint to dry a little so you don’t get any pooling or runs. 10 minuets should be plenty. It should tell you about recoat times on the can somewhere in the directions. I wouldn’t add any finish until you get the color even the way you want it. It will likely just enhance your problem.

Polyurethane brushed on would give you the best finish. You need to lightly sand the current finish with steel wool or very fine sandpaper to scuff it up, in order for the polyurethane to have something to "grab" onto.

Repaint light coats at a time and make sure you shake the can .

I think the best way to go is to sand as much of the ‘splotchy’ paint as you can. This should also rough up the original paint enough to where additional coats of paint adhere. Prime it with a good quality primer, and now your paint should do you right.