Guys,
Looking at available spray equipment and was wondering if different guns affect the lightness (weight wise) of the finish paint? The objective is to get a thinner coat with good coverage.

Does the gun have the impact or does the right reducer allow the paint to flow out. Windy had said the spray equipment wasn't that important as long as the surface prep work was done properly. It seems that if you have the proper temperature reducer that the paint will flow out and smooth. Only thing I was thinking is could certain types of sprayers would lay down a smoother coat and allow less coats with the same depth of coverage resulting in a lighter weight finish?

Best,         DennisT

Having good spray equipment is important for achieving a good finish with minimal material.  That does not necessarily mean the most expensive equipment, however.  The spray gun should have the fluid tip, needle, and air cap accurately made so that the needle will stay centered in the fluid tip.  The air cap should be made with a good pattern of air impingement holes to properly atomize the material being sprayed.  The diameter of the fluid tip should be appropriate for the material you are applying.

The material being sprayed should be thinned to the correct viscosity given by the manufacturer of the paint (dope, polyurethane, etc.)  The air pressure at the gun has to be appropriate for the material being sprayed and for the spray gun.  If all of these material and equipment factors come together correctly the material will be extremely finely atomized and will give the best finish for the amount of material applied. 

More specifically, for butyrate dope I would use a fluid tip & needle assembly of about 1.4 mm diameter with about 45 psi at the air inlet of the spray gun, given that the dope is thinned 50% to 60% thinner/dope ratio.  For automotive 2-part polyurethanes thinned appropriately (& they do need to be thinned regardless of the dictates in California by the EPA & AQMD) a fluid tip of about 1.2 to 1.3 mm diameter is good with about 40 psi air at the gun air inlet.  I have, for quite a few years now, used gravity feed HVLP guns with these numbers.  You can actually get a pretty good spray job and very good atomization with the Harbor Freight gravity guns, although I use the Sata Jet guns as they are a more versatile tool with all kinds of different size fluid tips & needles as well as a range of cup sizes to hold the material.  There is a pretty big price difference between the two with the Harbor Freight guns going for around $35 and the Sata Jet going for around $800.  Just depends on your needs & pocketbook.

Another frequently overlooked aspect of spray painting is the preparation of the air from the compressor.  The air has to be clean (free of oils, silicones, etc.) and dry - not spraying micro drops of water.  That is really important and can't be stressed enough.  Finally, it is really imperative that the spray gun be thoroughly cleaned after use, and that means taking the needle and air cap off and cleaning it in lacquer thinner, MEK, acetone, or the thinner used in the paint, particularly if you are spraying two-part polyurethanes or epoxies.  If you don't thoroughly clean the gun after spraying two-part paints you will end up with a trophy to hang on your wall (or throw away.)

Mark is well familiar with all of this & hopefully will come in here with more information.

What would you suggest to achieve this?  I've got one itty bitty regulator with a 1-1/2" diameter water trap.  I'm suspecting that's a bit small, but it's what Cheap Ass Tools had that day.

My dad did award-winning paint jobs using regulators with attached 2-1/2" diameter by 5" deep water traps.  I don't think there was anything more in the system than that, and as maintaining compressors is grotty and requires care to be safe, I ended up doing the routine bits of it once I was 14 or so.

I filter the air initially with a DeVilbiss 2 1/2" diameter x about 5" deep filter equipped with a drain in the bottom of the cup and this is mounted at the compressor.  Then about 30 feet of 3/8" diameter air hose runs from the filter at the compressor to another air filter/regulator that is mounted on a portable stand sitting on the floor that I can move around my shop.  This secondary filter is about 4" diameter x 10" deep again with a moisture drain at the bottom.  From this secondary filter I run about 10 feet of 3/8" very flexible air hose to the spray gun.  I drain these before and after every spray job.  I used to use a large silica gel dryer unit but I do that only if doing a full-scale airplane where the compressor is running for a couple of hours or more.

Then at the gun I have attached a pressure regulator that is used in conjunction with the flow control valve built into the spray gun.  Typically for dope I set the regulator at the gun for 45 psi with the trigger pulled and adjust the flow control until the spray pattern is right.  When the pattern is right the pressure at the nozzle will be right.

What I do to prepare the compressed air is probably overkill but then I don't have any air contamination problems.  All of the above is more or less the ideal way of doing it; however, I have seen some great finishes come from some poorly set up spray systems so it depends on how far you want to go with this.