Thin Valspar enamel with Naptha like the manufacturer recommends. Naptha dries slow and therefore the paint flows out smooth. No one knows their product better than the manufacturer so take their advice.
As for thinning, it's not all that critical. The way you apply the paint IS critical. Spray it on wet enough to flow but not so wet that it runs. You can control how wet it goes on by the distance you hold the gun from the target - the farther you get from the target the more thinner will evaporate before it hits the target.
The best thing you can do is a little test. 15 minutes of hands-on experience will get you familiar with the product. Valspar is good paint, been around forever and ain't going anywhere any time soon.
