Using the Powder by the Pound RAL colors suggested by Yondering only added .005" to my bullets. I have had no problems sizing them and shooting them unless I cured it improperly. By that I mean either I ran the oven too hot or too long or both. The test - smashing a test bullet to verify the coating stays on and does not crumble or flake, will give you some piece of mind. I have fired more than a few 45-70 powder coated bullets out of the 20" barrel without any leading. I have recovered bullets from the berm with the powder coating seemingly intact, though I did not fully inspect after cleaning the mud off of it.
I have had very few problems with gas checks. As per usual casting standards, if the base is crisp and sharp the gas checks snap on fairly easily. My failures have been when I have hard (bnh 22) double coated oversize rounds (.461 -.463) and tried to size them to .458. I mangled a few gas checks then. Heck, I tore my little Lee press off the plywood doing that. Once I tempered them with another trip through the oven (bnh 16) they sized pretty easily and the gas checks snapped on.
I use a heaping tablespoon and have powder left over in the tub. It gets on the bullet in the second coating.
I refer to use a one gallon paint bucket with lid but that is just a personal preference. It feels sturdier and the lid helps when my old tired hands slip. No more powder cloud calamities.
