Where do I start...All clears use reducer...some clears have the reducer built into the clear and all you do is catalyze it. Adding extra reducer to clear can cause it to "die off" as the solvents come out. This doesn't happen all the time and is dependent on how much extra reducer you put in. If your using a clear that doesn't call for reducer and you add reducer to it, you need to aware that reducers come in different strengths, and using a reducer not designed for the clear your spraying can give you the dying off situation I mentioned.
"Deadbodyman", I agree with what you said to a point, that you can't atomize clear if you can't get it out of the gun. If your spraying a heavy bodied clear and your getting orange peel, a 1.4 tip is going to atomize less than a 1.3. I've never had a clear that I can't get out of the gun, I have had clear that is heavy bodied to the point that I got orange peel...I solved the problem (for the way I spray) by going to a 1.3 tip. I had to move slower but I got less orange peel because I got more atomization. Base coat, being a thin product, atomizes much easier and can be sprayed successfully with different tip combinations. A lot depends on spray technique and product. If a 1.4 tip works for you with heavy bodied clear, that's what you should use. For me, I like my 1.3 for heavy clear.