[FONT=Verdana,Arial,Helvetica,sans-serif]Here's a good explanation of CFM and atomization. You can't go by tank size, you need to know the cfm of the gun, and compressor.
An HVLP gun requires more VOLUME of air to operate (the V in HVLP, High Volume Low Pressure). Now you may notice that your HVLP gun is adjusted at maybe the same PSI as an old conventional gun, around 50 lbs at the gun (many HVLP guns are set at much lower though) so where is the "Low" in PSI they are talking about? It is at the actual air cap where the air and paint come out. An HVLP gun has only 10 lbs at the cap while a conventional has upwards of 50! So the VOLUME of air (CFM -- Cubic Feet per Minute) is the key to proper atomization with an HVLP.
If you have a gun that requires 15 CFM you will need a compressor and plumbing that will produce that at a very minimum. There are HVLP guns that need as little as 7.5 CFM so you can get good results even from a smaller compressor. Air supply is a complete subject by itself, so let's assume that you have the air supply needed and move on to gun setup.
So atomization is the key, but why? Why can't you just lay it out wet and let it "flow", as an old painter will say. Picture a jar full of bb's; they will represent small, atomized droplets of PSPC. The gaps in between the bb's is solvent. Now picture a jar filled with marbles, they will represent large, poorly atomized droplets of PSPC. The gaps in between are, you guessed it, solvent.
If you apply your PSPC in large poorly atomized droplets, what you will have is a film full of solvent. This can and will cause slow curing, shrinkage and dieback (the loss of gloss in the hours and days after application).
So, now that we have learned the need for gun setup, how do we do it? Let's start with the fluid tip choice. The newer high-solids low-VOC PSPC products need to be broken up more, so a smaller fluid tip is needed.
Basically you want the smallest fluid tip that will still allow you to PSPC the particular part you are PSPC'ing, keeping the entire thing wet and in a fair amount of time. In other words, a 1.0 tip would be beautiful for clearing one fender, but would be lousy to paint a complete. The application would be way too slow, and the first panel would be way too flashed by the time you got back around to it. So you need to compromise -- a 1.3 is a great all-around tip, while a 1.5, though getting a little big, can get you by. If you read the tech sheet on the particular product you are shooting, it will have a recommendation for fluid tip size.
There are needs for other tips. For instance, when shooting polyester primer you may need as big as a 2.3, but for urethanes and epoxies, the 1.3 or 1.4 will work great.
As an example of the use of a 1.3 tip, I did a test once that proved the point well. I shot two panels of metal with a medium solids urethane primer. One was shot with a 1.3 super high atomizing top of the line topcoat gun. The other was shot with a 1.5 (or a 1.7 I can't remember) "hoser" primer gun. Three coats were applied and after a full cure (the one shot with the larger gun took MUCH longer to flash and cure by the way) the film thickness was measured. The one shot with the 1.3 tip was 2 tenths of a mil thicker! The larger gun laid out the marble sized droplets full of solvent, and when the solvent flashed the film shrank. [/FONT]