Disclaimer: longwinded technical blabla ahead [scared] - by all means skip it if you're not into it.
I agree with Alex - there's no such thing as a one trick pony when it comes to spraying.
Even if there are guns that are capable of spraying all liquids, you'll have to have a busload of nozzles and/or needles and/or aircaps to do so.
In my experience a good aircoat sprayer ( airless pressure pump with air support to encapsulate/form the spraycone and minimise overspray ) comes close to this elusive goal, but a professional aircoat system like for instance a Graco makes a Kapex with all bells and whistles seem a bargain....
Not just that, but you'll waste a hideous amount of paint when it's time to clean up. It's not uncommon to have a full quart or even more between container and nozzle - go figure.
I haven't tried a decent quality HVLP system, but I hear from colleagues in the GC and Painting profession that a decent 4 stage turbine system will do OK, but doesn't come cheap. It's main bonus being it's portability. The much advertised "big plus" of less overspray isn't percieved as significant whatsoever. If you want little overspray: go aircoat. That may even save you lots of finish / paint/ laquer as long as you're doing 10 gallons between cleanings.... I've tried some "conversion guns" and they couldn't hold a candle to a decent "regular" gun. Guess the "good ones" weren't among them.
Perhaps there's also a little jargon mingling here, for over here ( Netherlands ) "latex" strictly means wallpaint - a type of waterbased dispersion paint, non glossy, with at least thick yoghurt consistency, the main quality factors being it's opaqueness, glosslesness, scrubbing resistance and the ability to cover spots and discolourations in just one coat, if possible. It's only sprayed with airless / aircoat pressure type sprayers - they're even called "pumps" - it's the same type of equipment that's use to spray plaster.
On your side of the pond, I've seen things advertised as "latex" that are (semi) enamel type of paints, meant for use on wood siding. Totally different animal.
I'm with Alex in his preference for "old fashioned" spray guns. If you do have a decent compressor, it's by far the most capable gun for most finishes. Devilbiss are the bees knees in sprayguns, but there are other top notch manufacturers as well. I personally prefer a gravity feed cup, for I like the balance better and they're quite a bit easier to clean and on maintenance, but that's just me. They're less capable in odd angles, and can be somewhat tricky to use into tight spaces. It's just personal preference.
But the most important thing ( next to a good spraygun and a decent quality compressor - skip the oilless types as they're usually not very durable ) is quality of the supplied air, and that's tightly coupled to the quality of the aircleaner/reducer.
With a small or simple reducer there is a very steep relation between airflow and regulated pressure. The pressure drops significantly if the airflow goes up. This behaviour is especially severe around the zero airflow point. This means that a cheap regulator may allow the pressure in the hose to rise significantly during a static period ( no trigger depressed - no airflow ) and drop severely in the first moment of airflow. This period of overpressure will last longer with the length / volume of the airhose, and the elasticity of the material. A small ID hose with decent non-elastic braided reinforcement or made from a stiff material may diminish this behaviour somewhat, but it's very beneficial to have a supple hose, so as not to hamper your movement, and the best way to counteract this is at the source, being the regulator.
Pressure swings are very annoying, for the consistency of the paint delivery is one of the most important factors in a decent sprayfinish. When this "hysteresis" is too prominent, you'll waste lots of finish, for you'll have to start the gun next to the target, and wait for the cone to stabilise before you move on to it.
When you factor in that the venturi mechanism that delivers the finish in the airflow is far from linear, you'll see where this gets you: lots and lots of finish wasted, and lots of frustration. Hit the workpiece just a little too soon, and you'll lay on too much.
The tight control of regulated pressure in a membrane type regulator usually goes up with the diameter of the membrane, and with the length of the used coilspring. Of course the quality of the components like valves and such is very important, but usually: larger membrane, longer spring housing means better regulation with less hysteresis. Some high quality regulators use airsprings and pistons.
The larger the surface of the microfilter in the cleaning canister, the less oilmist and vapour gets through. It's very beneficial to put the aircleaner/regulator combo below the compressor if possible, and having a decent length hose between the compressor and the cleaner helps cool the air, so more moisture is separated. This may be a little less important spraying waterbased finishes, but in my experience, they're even more susceptible to traces of oil then an oil based finish is to traces of water.
Most compressors have a regulator on the tank, and quite frequently some sort of cleaner is integrated as well. While these regulators may be fine for your pneumatic nailer, they're hardly useable for spraypainting. Regulation hysteresis is very large, and since they're directly fed off the tank, there has been no opportunity for the air to cool, and it will contain a significant amount of moisture ( and probably oil as well ) after the cleaner/separator. From the tank, Most people want some "room to move" so longer hoses are favoured, increasing the "buffer capacity" AFTER the regulator. Besides, you'll want your impact wrench to run unrestricted, so a large ID hose is favoured - we want airflow, don't we?
For spraypainting, this is all wrong. You'll want a airclaner/separator that's about as detached as possible from the heat of the tank air and you'll want an air hose between the regulator and the spraygun that has the least volume, while being supple enough not to hamper your movement, and radially stiff enough not to expand under pressure at the same time.
Ideally, your compressor should be in a different room, for not only is the sound less annoying, ( and one can still experience an ever so little jolt in the movement when the compressor decides to kick in close to your ears ) but the cooling fan whon't have the ability to move massive loads of dust and half dried spray through the air as well. Try to get a good cleaner/regulator combo, and have it as close to the spraygun as practically possible.
A couple of years ago ( probably 10-15 ) as an experiment I've tried the standard regulator on a regular decent quality 2 cylinder 3,75 HP belt driven compressor, effective (!) capacity 345 ltrs/min or 12 cfm with a 25 Gallon tank on a standard summer day, tank pressure 10 bar or 145 PSI. It was connected to a professional spraygun using 50 feet of 1st quality synthetic rubber hose, 8mm ID, braided, rated for 35 bar or 500 PSI. This type of compressor is usually marketed as 18-24 cfm, and is about the largest compressor that's readily available for use on single phase power where I live.
When spraying an empty (!) gun on a piece of paper, non moving at a 10" distance a clear moisture pattern appeared in less then a minute.
The pressure drop was very noticeable, the gun producing a very distinguished PFFFffffffff........ with every press of the trigger. The first 1/4 second was effectively unusable, and it was very difficult to regulate the airflow and the needle. The amount of spilled laquer was enormous, and so was the residual spray ( floating particles of overspray that are half-polymerised can be just as damaging to your surface as dust particles - a risk using thinned synthetic laquers ).
It was effectively impossible to lay down a decent quality finish. I used synthetic alkyd based automotive laquer, properly thinned to the right consistency using a DIN viscosity cup. I opted for metallic cobalt blue as it's easy to spot ANY surface tension problems using metallic laquer and this was the colour at hand.
Using the same setup, but with a good aircleaner/regulator at the end of the hose ( fed directly off the tank, bypassing the "built in" regulator ) and a 6 feet length of 4mm ID braided hose to which the gun ( cleaned and dried ) was connected the picture was totally different. Consistent cone, no evidence of moisture, and hardly any pressure incosistencies.
In just under 15 minutes, I managed to collect a little under 7 cc's of moisture in the cleaner/separator. It was a warm summer day, but I wouldn't have called it humid - far from it actuallly. In fact, the "built in" regulator would have been delivering water instead of air about this point, but it didn't manage to capture enough moisture to be noticed when bled.
For spraypainting, I'd favour a decent quality cleaner/regulator setup with a crappy compressor over a decent compressor with a crappy cleaner/regulator setup every single time.
There's more to spraying then the gun or even the type of setup.
A very good spraypainter may be able to lay down a nice finish even if the total setup is a bit off. But mere mortals ( and I certainly am ) can use every single speckle of help.
For those of you interested: "Festool" was separated from the "Festo" brand.
Festo ( a nice light blue colour vs. Festool's lime green ) is still very active in industrial air supply and pneumatic control systems. They do have a nice range of cleaners/separators/regulators.....
End of longwinded technical stuff [embarassed]
Regards,
Job
