20 years ago I would base coat everything with Skull White straight out of the bottle. The last few years I have tried a few different things.
Now I DO own an airbrush. I have not been brave enough, or had time enough, to have a proper play with it. For similar reasons I have never used spray on primer (or any specific primer for that matter).
I have always just used a basic coat of paint.
Lately, with all my Bretonnian models and IG I have painted the last year or so, I have used cheap acrylic from the shops (see photo above).
I push out a measure on my palette and water it down a fair bit so it is quite runny. I then just larup it on with a decent sized brush. It does sit full of air bubbles and you do need to spread it around. In fact every time it looks like it is pooling badly and is going to be lumpy and horrible. But it never is!
It normally dries to an excellent, even, flat coverage with no loss of detail. I think you can see from these pictures that all the tiny bits and pieces are easily still available for painting, on the resin bases and on the plastic and metal miniatures.
Another advantage of this is you can pick whatever colour you want. Using a green undercoat for my IG and brown for my Bretonnian peasants just meant that if I did miss a bit later in painting, or there was a recess that was hard to get to, it would not be obvious at all. In fact on some of my minis I have cheated badly and used this 'undercoat' as a basic layer with no further paint on it.
I have not had any problem with my Vallejo or GW paints going onto these basecoats. (I know there is some bare metal showing. This is because these models had their slotta bit cut off as they are going on scenic bases and I was holding one end while undercoating the other. The missing bits will get undercoated when I do my first pass of painting which is normally the flesh areas).