in the coating world, 1 mil is 1 thousandth. Your conversion is correct. Why they do this, I do not know. lol.... I've asked the same question myself for years. (actually, I do know. It's a measurement between standard units which we use and metric where powder coatings were born,Europe circa late 1950's... why we have to be the walking conversion tables is the part that's unknown to me) When parts come from the fabricator to be used, they speak in terms of thousandths. When you give the product to your end user, they request the measurements in mils. It's just one of those things. There is however, a smaller measurement that you guys havn't used yet (and I doubt you will) and it's called the micron. A micron is simply one of the 25 parts of a mil. It goes like this.... 25 microns = 1 mil. 1 mil= 1 thousandth, and so on upwards. Very rarely will any coating exceed 30-50 mils in any application. As a matter of fact..... in certain coatings I've even heard a gasp come from the end-users' mouth when I told him it was 85 mils thick. Mind you.... the human hair is 3 mils thick. lol..... now you see what a tiny amount we're dealing with here. ( for those curious, the coating I just spoke of was called Ryton and it was intended to be a hy-build coating to be machined off with a tolerance of 17 microns. VERY close considering it was going on an induction blower for a dragster).
Anyways.... 1 mil is a thousandth. 'nuff said.
As far as your thickness goes, there is a tool out on the market that measures such a thing before you bake the piece. It's basically a scrape-gauge that you run along a part that's been coated to see what your final product thickness will be. There are also thickness testers out on the market that are *QUITE* expensive. It's not unusual to see them in the price range of an industrial gun itself. Figure is approximate to 3500 dollars, give or take which toys you add on. Now... I know what you are saying, "I'll never afford that!". Rest easy... there are alternatives to that price and I'm sure they are being sought out to be offered to you very shortly on this very website *wink* for both a thickness gauge and the scrape tester both.
I know I posted this process before somewhere... but for sake of argument, I'll give you all the cliff notes once again. Most any single coating you guys do will be in the neighborhood of 1-2 mils in thickness. (1.2 to 1.8 theoretically). Of course... some of the clears and so forth will be different from this amount, and some of the heavily pigmented colours will be less.... so this is a very general measurement. Let's take my infamous "widget" example yet once again (stop groaning please,lol) and use it for our purposes. Your widget is a 1/4" thick X 8"W X 14"L piece of sandblasted aluminum. Now.... the powder you are coating it with is a matte-black epoxy. Our widget even has round holes cut in it to make way for instruments to make things interesting. Heck.... it could even be something much like a dash insert for your old car,k? Anyways.... we hang the widget via a loop of wire to establish ground. Coat the widget until all of the sandblaste aluminum has *just* dissapeared. This is an educated guess that the coating has reached a cured piont of 1.2 mils. Usually, I go a wee bit past this stage so that even with a flashlight onto the surface of the widget will not reflect back the surface modification of the sandblast. All's we should see is black and it's powdered nuances (little glints of reflection but not a big "sparkle" of sandblast). We have therefore reached a targeted 1.8-2.1 mils. This is the point where we still see definition in our widget, yet.... everything looks amplified in a very fuzzy black manner. Hang in the oven and cure out at specified time/temperature, blah blah blah. THAT is your basic 1-2 mil pass. When fully cured, the widget should look much the same as it did shape-wise as it did before you sandblasted it with exception now that is is a matte black (natrually) and on the very sharp edges if you look closely..... it has an almost "rounded" edge to it. This is the coating that you are looking for in a first pass. Each additional coat will round the edges out that much further.
All in all... welcome to the boards and good luck with your powder coating. I hope that answers your questions...... Russ