Rob, I use the Ultra Matt to thin my paint, which might accout for patchy coverage at times, but I rarely ever use my airbrush. To be honest I hate cleaning it, so I win hands down applying everything by brush.
I have never tried a black undercoat, I have difficulty raising the chroma of what I paint using a grey cellulose car primer. When the current rattle can runs out I'll get a grey and a white from Halfords; using the same paints as I did with my latest two I have a achieved a much brighter red over an additional white undercoat. I'll post it once finished unless I break it again in which case it's probably going to have an intimate meeting with the wall.
I know many people swear by black underpainting so don't just take my word for it, give it a go if it doesn't work for you there is always a Dettol bath!
Cheers Simon