I'm leaping ahead with this one!
I managed to prime the model last night, so I've got the Sky undersides sprayed today.
I post shade, which I find more controllable. Using enamels I can also do it 'wet on wet'. If you over do it it is a simple task to do an overspray of the base colour to tone it down, which makes it an idiot proof technique, ideal for me.
It never shows up well in photographs, particularly mine, it's supposed to be subtle, but here is the underside anyway.
Varnishes also affect the final result, guessing how is all part of the fun! They always darken the way the paint appears, and this is a very small model.
BTW, I discovered that the round hatch on the underside, behind the radiator was how the armourers accessed the lower turret to install the ammunition boxes for the guns. Rather them than me!