Put the gas checks on before coating. I have done them both ways and they go on better if you check and size them before coating. Just make sure you seat the checks before sizing. Most people don't seat their checks. They snap them on and size. And then they wonder why they can't shoot them accurately or they get bad fliers.
I've also coated bullets after gas-checking them, with the same result as coating and then gas-checking. I did it that way because the bases of the bullets were too big after coating, and it was a PITA to get the checks seated straight. The coating adheres to the copper just like it does the lead. I also coated some bullets that I'd already sized and checked, to bump them up a couple thousandths of an inch in diameter. That turned out fine as well. Coating covers a lot of sins, and is a great tool for solving lots of problems.
