I have never used an HVLP gun before and am trying to figure out what compressor I need. I will be spraying small, metallic parts. The paint I am using recommends an HVLP gun with a 0.8mm tip running at no more than 25 PSI.
What size of air compressor (in terms of gallons and horsepower) should I be considering for this type of project?
All most any compressor should be able to handle an automotive sprayer, especially if it's only a few small metal parts. I doubt it'd even kick on half the time. If it does you just have to wait until it stops running (probably not even then).
The (very) important part is the regulator, set to 25psi max, above which the compressor's tank should have no trouble maintaining. If you're trying to paint a car, then you might want to look into a beefier compressor, because you'd want to avoid having to stop midway, to let it catch up.
More "gallons" means longer spans of not needing to run the compressor.
More "horsepower" means less time to catch back up.
Unless you're sand-blasting, really neither of these numbers concern you (at this time that is; go big if you're planing on buying stuff like nail guns in the future). A standard pancake compressor should suffice for just about any task except continuous sand-blasting.
Look at the gun's specifications. It will tell you how many cubic feet per minute, at what pressure, it runs at.
Then look at the compressor's specs. They will give several volume-ar-pressure numbers - the ones I've seen typically quote sustained volume at 90PSI for high-pressure tools and 40PSI for high-volume tools.
The compressor has to have sustained output equal to or greater then the gun requires, or you will run down the stored pressure in the tank and the gun will start misbehaving. Murphy's Law says this will happen in the exact place and time where it will make the most mess. Avoid y that situation; get an adequate compressor.
Note that there are alternatives.
Some HVLP systems are powered by turbines rather than compressors; these can't power any other air tool but are compact and quiet for their spray guns.
There are also airless spray systems, which just pump the paint into the nozzle; traditionally these weren't very good for anything much smaller than a wall but the latest generation can apply a furniture-quality finish.
There are "conversion" guns which you supply with high-pressure air which is used to produce HVLP air. These may work with older compressors that have bad low-pressure flow.
Remember that spray cans are also a kind of LP sprayer. Obviously they're more expensive per unit volume of finish applied, but you can buy a lot of spray cans for the cost of an HVLP system, and you don't have to clean them after every use. A handle that could to the can, with a simple trigger operating the spray, remotely improves the quality of the result considerably.
Finally, remember that there are other ways to apply finishes. I'm becoming a real fan of applying clear finishes with a padd rather than with a brush.
