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Hi Jeff, Opinions follow :o). Be cautious on the 3 cfm/hp "rule of thumb". If you are dealing with true hp it probably is reasonable, especially for older machines. However current hp ratings have been so botched by manufacturers that it may or may not be true. There are a number of 5 and 7-1/2 hp (so called) compressors out there that would not meet that rule of thumb. Take a look at your motor nameplate. It should have voltage and current ratings on it in addition to hp. A true 5 hp motor at 230 volts will draw about 21 or 22 amps at full load. If you are in that area, I would suspect you are close to the 3 cfm/hp rule. However, if your 5 hp motor is only 115 volt or draws only 15 amps if at 230, I doubt very much that you will achieve 3 cfm/hp...in fact you may be closer to 2 cfm/hp. In any event, I wouldn't get too hung up on the numbers at this point. If you were in a production situation it would be different. I am personally not a fan of HVLP. Since I learned on "conventional" guns years ago, I find using an HVLP to be quite slow...I feel like I am painting in slow motion. Of course I am used to using a remote cup pressure feed arrangement that puts down a lot of paint in a hurry. Gravity vs. siphon...oh, I don't know. I don't like either since I prefer the remote cup. That gets the weight of the paint (considerable if you're talking a qt) out of my gun hand and is truly all-position. Upside down to get the bottom of stuff is a snap. However, the remote cup requires a pressure fed arrangement. I have both gravity and siphon in my two "touch-up" guns. Either will tend to drip paint if you get too far out of position. I hate it when that happens, especially if it drips on your fresh paint! I will say that the gravity one is easier to clean...but is also a real pain to set down temporarily without a damn "special" holder. Gravity fed has only come about with the advent of HVLP. Before that everything was either suction or pressure. I suspect that reduced operating pressures in the gun passages with HVLP has made it more difficult to properly suction paint to the tip, hence emphasis on gravity feed. The 10 psi at the cap means that a maximum of 10 psig is present just under the air cap when the trigger is pulled. It is a legal requirement for HVLP. You will have no way to measure it without special test equipment. I wouldn't concern myself with this at all (unless you are in one of those California counties that require use of HVLP by law). Any HVLP gun should have stamped on the handle somewhere what pressure is to be used at the inlet to the gun to achieve 10 psig max at the cap. This is the number you need to be aware of with HVLP. For my HVLP "touch-up" gun this is 29 psig. I think that is pretty typical but it should be stated for each gun. A non-HVLP is likely to have inlet pressures up in the 40-60 psig area. You should always tee in a pressure gage at the point your hose connects to the gun (the inlet to the gun). You will need such a gage to set your operating pressure properly each time you use it. With the trigger wide open, adjust your wall regulator to get the proper inlet pressure at this gage...either the pressure stamped on an HVLP gun or the pressure specified on the material tech data sheet of your paint for conventional guns. Do NOT try to set it to any 10 psi. That is at an HVLP cap and can be substantially different (lower) that at the inlet on the same gun. All reciprocating air compressors exhibit a drop in cfm capacity with increasing pressure. It is due to the changing volumetric efficiency as the discharge pressure goes up. Single stage compressors can show a significant drop at higher pressures. I once had an old single stage machine that provided 7.8 cfm @ 40psig and 6.3 cfm @ 90psig. That type of drop with higher pressure should be quite typical. Two stage machines will show a much smaller drop because the compression ratios per stage are significantly less than a single stage, yielding higher volumetric efficiency. If you wish to compare compressor capacity with gun capacity, you need to allow a liberal amount for pressure drop from your tank, through your wall regulator and then to your gun. My suggestion would be to use the compressor capacity at 90 or 100 psig...definitely not 40. If you are using say 30 psig at the gun inlet, you should figure on it being necessarily much higher back at the compressor, which is what your compressor is rated on. Personally I would not recommend getting into expensive high-end materials and equipment at your stage. Professional body shops that do high-end paint jobs on a continuous basis use materials and equipment that could make the DIY go broke trying to duplicate. Unless you want to compete with that $5000 paint job on a Mercedes, my suggestion is that you do not try. Tractors do not have large flat surfaces and the sheetmetal is only a portion of the total. There are usually a lot of cast surfaces that you will never get to a mirror finish, realistically speaking...nor should you try. If you are considering doing your tractor piecemeal, you might consider only a touch-up gun. Matter of fact you should consider a touch-up gun anyway. There are so many small pieces on a tractor job that having only a regular size gun is unnecessary overkill. The direction you take should also be guided by safety considerations. Unless you have (or are planning to get) fairly expensive supplied air respiratory equipment, you should stay away from urethanes and hardeners in enamels, including clears. That is, unless you wish to play fast and loose with your own health. That pretty much leaves the synthetic enamel that you mentioned and automotive acrylic enamel, both unhardened. That, in turn, negates any possible color sanding and buffing...you would destroy the gloss in the process. So forget any corrective action after the spraying is done. As woodtick said, practice and practice some more...on vertical sheetmetal. You need to develop the knack for putting the paint on without runs or excessive orange peel...a fine line...because you will have to live with the result with the unhardened enamels. It's not hard. It just takes a little practice. I also would not be greatly concerned about your compressor capacity. Since you are not in a production situation you can make do with less air from the compressor than the gun needs on a continuous basis. The fact that your tank is 60 gallons gives you a reasonable reserve and will assist in prolonging the actual "trigger on" time. Certainly if you attack the matter piecemeal you should not have a problem with any reasonable regular size gun. I just don't think it's a problem you need to be too concerned with at this point. I would recommend for now that you get a $40 or $50 knock-off touch-up gun, some synthetic or acrylic enamel, use your existing compressor and practice. You can always move up to higher quality equipment later without much, if any, wasted costs. I did most of the sheetmetal piecemeal on my AC-B last summer with just a touch-up gun. It was simply too much of a bother to fire up the large gun for what really was several separate pieces, none of which had large unbroken surface areas. I wouldn't try it on the hood of a full size car, but that's a different situation altogether. For your trucks, and if your tractor has large unbroken areas, you would want a full size gun. Get whatever your budget can afford and don't get overly hung up on brand names. You can achieve quite respectable results without trying to compete with the high-enders in either materials or equipment. Rod
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