You cant spray Flake through an airbrush, certainly not a conventional airbrush. Metallics, yes sometimes, but not Flake.
Usually, Flake is applied either in an inter-coat carrier or in one of the clear coats, having been mixed in.
There is a device known as a Flake Buster, which you can copy by modifying a regular air blower type attachment, so that it sucks up flake from the pot and sprays it out.
Also, if you do apply flake manually onto clear, you need a HEAVY coat of clear, so that the flake mostly sinks in. Then you need to apply more clear until it is buried, BEFORE you attempt wet sanding. Most flakes are not coloured all of the way through, so if you sand them, then they will show up as white specks in your finish. House of Kolor flakes are the same colour right through.
Personally, the links to the Tele forums that were posted...the finish is awful. If they really want that thick textured finish, fine, but it's not my cup of tea and not how flake should be applied.