So the gist of it is as far as spray coating canvas, in my opinion, is bigger nozzle size is in general better, up to the point you start getting runs. It is really that simple.
Now you can stop right here and will have gotten all you really need to know for the purposes of this discussion about spray coating canvas. Below is some of the rest of the story. Proceed at the peril of your sanity.
Basically overspray is a function of air pressure and nozzle size needed to atomize to your desired or needed particle size, the desired volume and type of material you wish to coat with. The cap size is a basic governing factor in how much fluid you deposit at what particle size at a given air pressure, in a pressure fed fluid delivery system like the T5070 gun. Gravity fed guns are a bit different in that they arent tied as directly to air pressure but still constrained by cap size. In general over spray is more dependent on the atomizing air flow and how fine the particles you need/wish to get in the delivered material. Finer particles mean more overspray and we dont need particularly fine or uniform particle sizes to coat a canvas.
In many canvas coatings there are leveling agents (viscosity modifiers) that allow the material to end up nice and smooth even when the material is poured and or globed on, then smeared around with a roller brush until in some cases it gets somewhat tacky/dry (that crackly sound the roller makes) and then leaves relatively huge peaks and valleys on the coated surface, they will level the surface given the appropriate further drying times.
The T5070 gun is what is called in the automotive trades, a finish gun; it is capable of producing very fine and very uniform droplet sizes, which is desirable for coatings whose liquid vehicle is some type of VOC (volatile organic component/carrier) as well as being a multi-part chemically reactive cocktail, think very thin runny colored epoxy glue. So once again, basically the finer the droplet size the more overspray. Now add to this the patterning air that comes out of the wings of the cap to produce the fan pattern and you have a very fine mist capable of being blown around and floating everywhere even given the very smallest of air currents, in particular if you get too aggressive with pushing the spray pattern around trying to get a wide fan. As well there is the add fact that many people have a tendency to spray way too far away from the surface they are trying to coat. I have seen some trying to spray at distances of 18 to 2 feet, when 8 to 12 is really optimal with a good gun that is correctly setup. This leads to loads of overspray.
So if we were coating an extremely smooth surface, and we are obviously not, with as close to zero surface imperfections as humanly possible (think a well prepped auto surface ready for finish painting) as well as using a very fast evaporating vehicle (VOC) and it is also an actively reacting chemistry, this is a great gun. But, even given the fact that this is called an HVLP gun and does reduce the amount of VOC required to get your coating material in a very uniform very thin layer where you need it, you would still normally be using this gun in a down draft spray booth because it still produces a substantial amount of overspray, because of its inherent design parameters. It just does so with a bit less of the VOCs that the government wanted the automotive industry to reduce, or do away with, which brought about the HVLP gun designs in the first place.
All this is a very long winded way of saying you dont need a $300 to $400 ( gun alone, more with source) very high quality automotive finish gun to lay down a material that can actually be applied, acceptably, with a roller brush, at least by most people most of the time. Which when we do use it in our application, then requires a good bit of dilution of the materials we use to get it to even go through the gun in the first place, one designed to spray quite thin very fast drying materials in exceptionally thin exceptionally uniform layers in the first place! It doesnt seem to make a lot of sense, now does it?
A $50 gravity fed HVLP primer gun, designed to put down a heavy load of thick material quickly with their larger nozzle size will provide all the desirable qualities of spray coating, faster, easier, greater control of application, more uniformity of the applied coating, use of less coating material (if you find that desirable, some liking that foot deep coating look, which can be had in far fewer coats using a large diameter cap), greatly diminished chance of damaging a delicate surface. All this while greatly diminishing the negative side effects of this application method, that of overspray in our case, or thousands to millions of tons per year of VOCs being rejected into the atmosphere in the automotive and other commercial coating industries using VOCs as a vehicle.
Increase cap size, all else be equal will give three things, more volume of material delivered per unit time, at a larger if less uniform particle size (which in our case for the most part doesnt matter because of the leveling agents) with less overspray because the particle sizes are so much larger and less buoyant in air.
Now you know, some, of the rest of the story, with sanity intact hopefully.
Later Larry