I've been slinging paint on objects for at least a couple of weeks, now, and I've NEVER heard of anyone spraying coats of REDUCER on AFTER they've sprayed their paint unless you're working with Laquer's with an airbrush and you're looking for some of those funky special effects. Acrylic Urethane is a Single-Stage paint. That means when you spray it, it glosses over without needing a clearcoat over the top. I have not worked with anything labeled as an Acrylic Urethane Enamel before, but that doesn't mean that someone doesn't make it. It's probably the same thing.
As far as spraying it, if it requires a reducer, the side of the can (which you should ALWAYS read! Even if you've shot the stuff 100 times before) should tell you what to reduce it with, and at what proportions. Most paints today are reduced 1:1. That is; one part paint, one part reducer. And by reducer, I mean THE REDUCER THAT IS MANUFACTURED BY THE SAME COMPANY FOR THAT PAINT!! A 5-gal can of Laquer thinner ($4.95)isn't a substitute for a 1-gal can House of Kolor Medium Urethane Reducer ($35.00) when you're spraying HOK's Kandy Tangelo Orange.
Most paint supply houses have nifty little clear plastic measuring cups with MANY different mix-ratio's on the side. They can show you how to use the cups properly.
IF you spray your Acrylic Urethane and then spray straight reducer on afterwards, the reducer is going to re-liquify the paint and it will RUN OFF OF YOUR CAR. I don't mean flatten, I mean RUN! Reducers are made to thin the paint so that it is easier to spray through a spray gun. Some supply houses allow you to purchase gallons of "ready-to-spray" paint that already have reducer in it.
When you read the side of the can of paint, see if anything else is required in the paint...like Activator or Hardener. Most of todays paint colors don't require these additives, but most clear-coats do. If your paint requires it, purchase the correct stuff and measure it correctly.
Anyways, bottom line is....DONT try to spray straight reducer on over the top of paint. Unless your spraying Spies-Hecker, it's just going to peel the paint off.
Also...I may be wrong, but it seems like you may not have done your homework on how to paint whatever your trying to paint yet...go get a book and read it 4 or 5 times. (I recommend John Kosmoski's Custom Painting Secrets <-I think that's the title> It'll take away allot of headaches later.
Acrylic Urethane Enamel is a single stage paint, its offerd by many companies like Kirker and hundreds of others. The last coat is reduced by 75%, this is a last ditch effort to smooth out the paint for beginners or hobbyists, many old school pro's do it as well. The paint has been thinned so much its nearly impossible to lay down anything but a smooth last coat. This is normally done at least 10-15 minutes after the last correctly reduced coat. To soon and you'll ruin the previously sprayed paint.
Pure reducer will just ruin the paint. Before you jump into this and waste your money listen to the 'hopper and read up on painting, the properties of paint and anything else you can.