While I'll admit the Army Painter sprays can be inconsistent, I think out of maybe 20 cans (of various colors--I have most of the range that I used before I had an airbrush when I was doing commission work for fast basecoating) I've had one can that was bad, and it was an internal pressure issue, not a grainy issue-- it came out of the can like silly string.
If you have problems with grainy sprays, be sure to spray in a cool, dry place. Shake the can for 30 seconds before spraying, and hold the can upside down while spraying. Spray in short, two-second bursts, rather than one heavy pass, and spray no closer than 6'' to the model. Somewhere in there, you should solve whatever's causing the poor finish.
Now, all that being said, if you own an airbrush, it's a lot better to just spray the model with your preferred primer color and then apply the bone color with the airbrush. Airbrushes are becoming increasingly common (I know I was VERY resistant to getting one, but I wish I'd done it years ago now that I have one), but not every hobbyist has one yet. They're certainly less useful for Fantasy models than 40k models, admittedly, but still a good investment regardless of what you paint. That being said, DO NOT buy the Games Workshop spraygun. It's a bad siphon-feed airbrush and only can use their canned air-- the odds of it working correctly in the first place are hilariously low, and if it does, it's cheaper to buy a Masters brand gravity feed airbrush and an actual air compressor-- it's about twice as much upfront than getting the GW spraygun and enough canned air to operate it for any meaningful amount of time (i.e, roughly $100) but it'll actually work.