Constructing your own oil/water sprayer
As someone who's currently trying to construct a sprayer for a sort of performance art thing, my recommendation is -- don't bother.

There are plenty of pump sprayers available at kitchen stores that you can fill and use.

There are two basic styles -- the ones where you squeeze a trigger and stuff shoots out with each squeeze like some cleaning products, and the ones that you pump up first and then press the button to release like an aerosol can.

I like the trigger ones for water (for keeping near the grill to deal with flareups).  I like the aerosol-like ones for oils, as I get a finer spray for popcorn, and I can still lay down a decent coating in a pan for cooking.

I do not like the stainless steel aerosol sprayers -- you want a clear one, so you can see how much oil there is in the container, and you don't overfill, as you need to leave quite a bit of space for the pressurized air.

You should also look for ones where the tube goes deep into the container -- if it stops 1" from the bottom, that means you can't use it when the oil or liquid is low, and you have to refill it more often.

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I should also note that Pam and other spray oils have an additive in them that makes it so they won't bead up after you spray it, which means it'll coat things better ... misting it with oil works pretty well, but it'll start beading up if you leave it after doing it.