Yes video 5 (very) briefly covers it. I use a very thinned down black paint, apply a coat, let it dry. Rub it back with scotch-brite or some wet and dry paper, removing most of the paint but leaving a couple of spots (approx 25% paint coverage let’s say) then apply second coat of thinned paint. Those previous spots of paint in the first layer will now turn black, and the new layer will be grey. Rub this back lightly to reveal some fresh aluminium, especially on edges or around holes.
I also use the spray gun on a low pressure with pure thinners in it to ‘spit’ thinners, and dab rags with thinners on them on the frame. You have to just play about with it.
If you don’t like it, just wipe everything off with thinners and start again. Works for me but please experiment! Let us know how you get on.
I will post a dedicated video I promise, just need to get the build step by steps finished first.
Thanks!