I would definitely go with gravity feed. What looks like a little cup is actually 5 to 10 minutes of paint and that is a long time if you are really focusing on doing something detailed. Gravity feed will work at 12 to 15 psi and syphon feed needs 35 to 45 psi just to such paint out of a cup or bottle. So gravity requires much less of an air compressor or really saves on how much it operates, hence the electric bill and how much big air compressors cost.
Just watering down acrylic paints can become a problem, some of them mix with water and some of them just become a goo. I use createx air brush paint right out of the bottle. For just becoming familiar with what it does, or how fine you can get, you can use india ink. Usually air brush paint you just use out of the bottle and it is a bit more as the pigment is ground much finer than cheap acrylic paint. Sure, enough thinning (some require a touch of isopropal alcohol) and enough pressure and you can shoot anything. This isn't control.
With higher pressures you get spatters on your work when getting really close and fine. If you intend on doing fine work, the 15 psi range is better than blowing something with 45 psi or like Tshirts are done at 60 psi to intentionally blow through the fabric. Fine automotive work require 12 to 15 psi and the best quality paint.
Just starting out gravity feed will save hundreds of dollars in experimenting with what they can do and what you can make money with only to find out you needed a gravity feed anyhow.
I stopped buying things once I got Iwata air brushes. Either an Eclipse or HP-C model are great. It used to be they weren't even available in stores or at trade shows. And they used to be super expensive.
I see so many people buy all sorts of gear like multiple air brushes, little manifolds to have 8 hooked up at one time, a bunch of bottles and 6 ar brushes, that special compressor that isn't so special after all and get $1200 into things. You just need one good air brush and a good regulator on an air compressor that is a real air compressor. I have a regulator and a bunch of valves that are actually air tool throttles at the end of the air hose and a regulator there, then the air hose to the air compressor is a reservoir of 100 psi regulated down to 15 psi to 20 psi. I have portable compressors but only use them for cleaning air brushes at the sink in a different part of the building. It is worthless for quality work.
Pretty much every shop I work at has an air compressor so I just hook up with my own hoses and regulators and not trust theirs to be accurate or working at all. The compressors all have water traps and using your own hose means no air tool oil screwing up your paint.
I set up to do motorcycle paint jobs that can be very fine detailed pictures represented in small spaces or large flowing patterns and I find the gravity feed does it all. If you watch someone do it, they go over the same area many times until the color is deep enough or the lines are subtle enough. Blasting something with mass quantities of air and paint don't get it unless you are doing stencils and fabrics, going for something bold and quick.
Then any airbrush still has some limitations for detail. Sometimes I cheat and do the painting in fine artist brushes then give it an over spray or shadow with the air brush so it looks air brushed. Then it is a mystery of how did that guy get that from an air brush. You have to be smarter than the bowl of paint.
Just starting out with the paint you are going to be using is also best. Going cheap will lie to you on how difficult air brushing really is and how many clogs you have to clean out of the gun or off your paint job. Every material comes out different, uses different air pressures and result in different levels of detail.
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