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If you want a quick summary of how I would choose a spray outfit, keep reading. But for a better understanding of the different spray systems, see the separate page on spraying finish and other application techniques.
Okay, I have a bunch of spray options that I own and have used. But the technology of spray guns has advanced, so the simple answer of "same as Charlie" may no longer be the best answer. I have talked with friends who have newer solutions, and adding their experience to mine, plus some research, here are my recommendations. I will try to identify the solutions I have actually used.
HVLP is a key concept. High Volume (of air) at Low Pressure (less than 5 psi at the tip of the gun) can - with a good gun - atomize the paint into a fine mist. The the low pressure moves the paint mist to the surface more slowly - with less bounce of the material off the work piece and less overspray. The high volume of air keeps the paint moving in the right direction. The result is SO much less overspray and waste that many jurisdictions now require HVLP - you can no longer legally use your antique spray gun.
The original HVLP solutions required a "turbine" to produce the large volume of air, sounding much like a vacuum cleaner. Actually to get the volume/pressure required, 3 to 5 compressors are often run together - 3-5 stages - with one feeding the next. And with the relatively low pressure from the turbine, a large hose is required to move the air to the gun. It looks like a ¾ inch garden hose but is special because the recently compressed air is hot enough to destroy an ordinary garden hose.
Boyle's law frequently comes into play in this discussion. In simplest terms, if you have a given amount of air, and double the pressure, the volume is cut in half. If you have a 10 gallon tank of air at 150 psi and let it out at atmospheric pressure (15 psi) you will release about 100 gallons of air. Scientists want to talk about having to keep the temperature constant, etc, but roughly pressure times volume is constant - double one and halve the other.
If I already have a compressor, why can't I use the 120 psi air coming out of the compressor, move it through a ¼ inch regular air hose (since it is at high pressure Boyle says the volume can be far smaller), and reduce it at the gun? Convert the high pressure spraying that historically used high air pressure at the gun to an HVLP system? Read on.
Years ago I bought and loved the $100 Porter Cable PSH1 gravity feed HVLP (High Volume Low Pressure) conversion gun. There were many look-alike copies, some of which worked better than others. It is called a conversion gun since it runs off a high pressure compressor, converting the air to low pressure, rather than directly from a low pressure turbine. Not shown in the picture is the pressure reducer/control that comes with the gun and attaches to the bottom, where the air enters. I can leave my compressor at the usual 100+ psi or so, and reduce it to 20-40 psi where it enters the gun, then the air expands as it goes through the gun until the pressure is reduced to about 5-10 psi at the tip. I used this gun as my primary spray gun for years, and have happily recommended it to many people. However, it appears that Porter Cable is discontinuing this gun - Amazon says they don't know when or if they will be able to get more.
This gun, and others like it, runs off my cheap compressor that produces about 4 cfm (half the volume required by the gun) but has a 13 gallon tank. That means that I can use air from the tank when the gun "uses" more air than the compressor produces, and when I pause, the compressor will continue to run to replace the air in the tank. Overall the tank allows me to keep the spray trigger pulled an average of half the time (since I am using air twice as fast as the compressor produces it). This is not a problem - I can go for 5 minutes or more of intense spraying before I get ahead of the 13 gallon tank. If I use my small compressor with the 1 gallon tank, I have to stop several times per minute to allow the compressor to catch up. A larger tank would allow me to spray longer before I had to stop to allow the compressor to catch up... average over a longer period, but the large tank requires a lot of time to fill up before it is ready to use.
PSH1 or equivalent? Many vendors offer competing guns. Harbor Freight has several similar looking guns, sometimes for as little as $25 (but you purchase the $7 pressure regulator separately). I know people who are happy with the $75 copies. I tried the $25 copy and the results were far from as good as the PSH1. Apparently the holes in the air-cap/nozzle are not quite as precise as in the name brand gun, and the atomization isn't quite as good, but it works "okay." Incidentally, I have never been happy with a spray gun that cost under about $100.
Initially the HVLP "conversion" guns were considered a compromise, but the technology has advanced so that today's best guns are conversion guns - if you have a good compressor, a turbine is no longer required. The newest guns recognize you may be using a smaller "contractor", "pancake", or "hot-dog" compressor, so there is a whole lineup of "LVLP" (Low Volume Low Pressure) guns that use about half as much air as most of the HVLP guns (although many are just advertised as HVLP guns that require perhaps 3-6 cfm rather than 12+ cfm). And since there is no standard about what constitutes LVLP, some of the conversion guns that actually use a lot of air have the LVLP label, thanks to the marketing folks.
The next technology is Reduced Pressure - but not as reduced as HVLP. The definition of HVLP requires 5 psi or less at the tip; reduced pressure may have tip pressure more than 5 psi, but may have better performance - atomization and coverage - than HVLP, through refined technologies. Some vendors call it RP, other vendors call it Trans Tech for high transfer efficiency technology. CATech calls is CPR - compliant Pressure Reduced (although many of the CATech guns come with a separate air cap that goes back to HVLP if local regulators require it).
To select from the many outstanding guns on the market (most are now conversion guns) is a huge task. I have a friend, a professional furniture refinisher, who loves his C.A.Technologies Jaguar, but I have not used one. My Brother-in-law bought a Jaguar gun after his PSH-1 died, was horrified at the price of the Jaguar with all the options, but when I visited him, he raved at great length about how wonderful the Jaguar gun was. He described it as creating a cloud of atomized finish that moved slowly to the workpiece (I haven't seen it, but if he says so!). If I were to buy a new conversion gun, this would be it. See Jeff Jewitt's web site or Spray Gun World's confusing web page for kits, or look for the gun only at Spray Gun World.
If you need a cheaper gun, but want something reasonably good, my favorite Porter Cable is no longer available. Therefore I went looking for something to recommend and found the LCFM HVLP gun at Spray Gun World. I would buy it with the 1.4 mm and 1.7 mm tips (it comes with your choice of two), and the $10 regulator, for a total cost of $134, delivered, at the time I wrote this. NOTE that this recommendation is based only on web research, not on personal use of this gun.
If you have time, I suggest you explore the options at Spray Gun World. Look under topics such as woodworking and look by brand. When you refine your questions, send Spray Gun World a message and they will call you back - be sure to include your phone number since they respond by phone, not email. They are knowledgeable, and sell so many different brands that they appear to have less bias towards a single vendor.
I also have a Turbinaire 3 stage turbine, pressure cup HVLP gun, variable speed, chosen because somebody owed me money and offered this. It originally cost about $700, and has worked very well for quite a few years. However, to my dismay, Turbinaire only sells parts through dealers, and most of the dealers are "closing out" that line, so I cannot get the repair parts I need. The factory admits they no longer stock the parts for their "obsolete" models, meaning you cannot maintain their equipment, so the simple answer, I wouldn't touch the Turbinaire brand. I have had to trash their several hundred dollar gun because I couldn't buy a $50 replacement spray tip. I bought a replacement gun, made by C.A.Technologies, (or CAT, Coatings Atomization Technologies) from Spray Gun World, a most helpful company, with wide variety of products for all types of use and budgets, and great prices. In fact the CA Technologies gun was so good, and inexpensive, that I bought a second unit so I could use sealer in one and finish in the other, or similar combinations.
A turbine can have 1 to 5 "stages." Most vendors use the same manufacturer for the actual turbine, so the stages are the number of units put in series. A single stage turbine is used in very cheap systems - the atomization of the finish won't be as good, so the results will be inferior - looking good if you have nothing to compare with, but still inferior. A three stage turbine is considered sufficient for serious amateurs. A four stage turbine is considered minimal for professionals, and will atomize heavier finishes without thinning. You may even be able to spray latex paint with a 5 stage turbine.
Turbine units are light weight (no big tank) and thus portable. Most turbines are sold as systems with a turbine, hose, gun, etc. are available from several vendors. Earlex systems are not professional grade (some have called them single project systems - they may not be that bad, but they are not for heavy use). Using Spray Gun World prices, Fuji complete turbine systems are available from $565 (low end 3 stage) to almost $1000 (high end, sound proofed, 4 stage). Apollo systems start higher than Fuji and can cost $1500, but have a great reputation (check noise level, since earlier systems had a reputation of being very loud).
Breaking news... A friend who does amazing work recommended the Sprayfine 4 stage system from Turbine Products. He has been using the system for many years and loves it (but not the included gun). Amazing part is that you can buy the turbine (3 or 4 stage) separately, and buy their air hose, and not have to buy their gun. Put a ¾ inch ball valve in the air line for additional control.
I had a cheap single stage turbine system, but sold it. If your material is perfect, the atomization (making a mist of paint) is okay, but it isn't very forgiving. Since you have minimal air volume, and many of the critical gun parts are plastic, I have heard these systems described as for "single project" use, where the spray quality decreases as the material wears the plastic parts. I kept the Porter Cable gun, and the Turbinaire 3 stage system, and sold this one.
If I were buying another turbine system, I would seriously consider getting a 4 stage turbine. Some of the turbine units are notoriously noisy, others are quieter. Most turbine guns have a pressure cup (below the gun) but some are starting to offer gravity feed systems (with the cup above the gun). If you will ever be using water based finishes, be sure to get stainless steel needles, nozzles, and fluid path - all the parts that touch the finish. My current turbine has variable speed, but you can use a garden hose ball valve if you want to reduce the air volume, if the air valve on the gun is not sufficient. I do not have experience with a particular turbine brand that I can recommend, but the primary players seem to be Fuji and Apollo.
Most of the time I use the turbine gun (with the material cup at the bottom in my system), either because I am spraying a larger item and don't want to wear out my cheap compressor, or because I can turn the volume down for detail work. When I am doing larger items, it is easy to keep the paint cup at least half full, so there is little concern about tipping the gun, but when the amount of paint in the cup gets low, the gun gets really cranky about what position it is held in, therefore making it harder for me to use the bottom feed guns - to get all the finish out of the cup. It also has a longer material path (the tube that goes to the bottom of the cup) so it takes slightly longer to clean.
Generally I prefer a gravity cup, such as on my Porter Cable conversion gun. The shape of the bottom of the gravity cup allows me to tip the gun into almost any position, and with the short material path, I can use the last tiny bit of material (important if you have some tinted finish you want to use up without mixing extra just to "prime the gun"). The short path also makes cleanup slightly easier - not an issue with most water-base finishes, but makes me choose the gravity feed gun if I am facing a tough cleanup. Some pros do not agree with my preference for gravity feed.
If I had to replace my spray system today, I would look closely at the Fuji 3 or 4 stage turbine system - preferably 4 stage with the 1.4 mm tip, for $689 or $820 complete with free shipping, or the Sprayfine system mentioned above from TurbineProducts.com
I have an airless sprayer that I use for latex paint, but it does not atomize paint as well as an HVLP sprayer. When I was buying it, the salesman immediately stopped trying to sell me one, when I mentioned possible use on furniture.
I have, and no longer use, a conventional gun with a large paint pot, a conventional gun with suction cup, and a conventional touch up gun. Why don't I use the non-HVLP guns? WAAAAY too much lost material. This old technology uses a relatively high air pressure to get fine atomization of the paint, but that makes the paint move so fast that much of it bounces (or never reaches the target) and becomes overspray. I estimate that I use twice as much material to spray an item with a conventional gun as it takes to spray the same item with an HVLP gun, and the better HVLP gun gives finer atomization leading to finer finish. Reducing the volume of air on my turbine gun allows me to hit as small an area as the conventional touch-up gun, so I don't even use that one.
The first conventional spray guns required a compressor with relatively high air pressure. When HVLP was "invented" it assumed a turbine with a high volume of relatively low pressure air. Later the conversion guns were invented, to allow HVLP performance with existing compressors. (I leave the hose from the compressor at 100+ psi, then use a pressure reducer at the inlet to the gun, down to about 20 psi, then the gun cuts the pressure further to (officially) under 5 psi at the tip.) The first conversion guns were a poor compromise, but have improved, until now some experts claim that the best guns are conversion guns, now better than the turbine guns.
A turbine has a few advantages: The air delivered to the gun is warm and relatively dry. Some prefer not to spray with warm air, but I prefer it to cold, which can cool the work, slow the curing, and even condense moisture in the finish (blushing). The turbines are far more portable than a large compressor. They require less power - any I know plug into a household outlet. And you don't have to wait for the air tank to fill before using a turbine.
A compressor has it's own advantages: The air delivered to the gun is cool, (since it expands as it leaves the tank), but the cooling can lead to condensation - moisture problems in the tanks, air lines, and guns. The small high pressure air hose is far more convenient than the much larger hose (garden hose size) used on a turbine. There are many uses for compressed air in the shop, which justifies a better compressor and larger tank. Portable compressors are heavy, especially with "good size" tanks. A properly sized compressor may be 5 hp or more, which will require a special electrical circuit; a smaller compressor will not support continuous spraying.
One other factor nobody discusses. Many guns (including the Porter Cable) have about 8-10 tiny holes for air to come out of the cap around the material nozzle. This is presumably to improve the quality of the atomization (although my turbine guns do not have the small holes, and work great). I spray outdoors, so a gust of wind can blow material back into the gun and plug those tiny holes. They are a pain to clean (the holes are too small for most cleaning weapons). To me, a new gun gets extra points if it does a good job without those tiny holes, but experts tell me the little holes really improve the spray quality. Recently I have been leaving the aircap in a jar of denatured alcohol between uses - the alcohol keeps water base finishes from curing, which presumably will help keep the holes clear.
Most guns can change the nozzle size (but you probably also have to change the size of the valve needle at the same time.) All my spraying has been with a "standard" size needle for the gun, typically about 1.3 mm. Smaller nozzles are required for fine stains; larger nozzles are required for thicker finishes such as latex. I hear auto finishes are thinner, and often use a 1.0 mm tip.
Oil bathed air compressors are far quieter and longer lasting than "dry" compressors. But they pass minute amounts of oil into the compressed air. As air is compressed, it heats, and when it leaves the tank is expands and cools, so there is often condensate water (dirty rusty water) in the air tank. Therefore experts recommend a filter close to the spray gun (in the spray area rather than at the compressor, because of those dirty pipes). One expert recommends the disposable in-line Grainger 6ZC63 filter, package of two for around $20. The desiccant turns from blue to pink when it is time to change the filter, but one user notes that you can dry the desiccant and reuse the filter, since the primary contaminant is water.
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