Undercoating....

[SteveBurt]

Brush on Liquitex artist acrylics. Normally use Bronze Yellow (a sandy brown colour).

[Orctrader]

Brush on Humbrol matt enamel...

Me too.

[Bearwoodman]

I favour the Citadel spray primers that are designed to go with their contrast range (there is a pale grey "Grey Seer" and a more creamy "Wraithbone").  This is not just because I like to use some contrast paints on most of the figures I paint; I find these primers' coverage is very fine, matte and smooth, and I find the grey colour makes detail easier to see compared to a white or black undercoat.

I recently had to paint up a batch of skeletons in a hurry and for them I tried a zenithal prime (spraying Wraithbone over Tamiya matte black) and that seemed to work very well. As Wirelizard says, this is a very effective pre-shading step for very little effort, particularly if doing a number of figures at a time, and I think I will be using this method going forward.

[War Monkey]

I tried spray on Auto Primer and that was okay. However, I had to wait for warm weather to spray anything, and I don't have the space for a spray booth at this time as well, living in one of the colder states in the US has it limits

So, I change what I would use, I switch to Vallejo Air Brush Primer, however instead of fiddling around with an air brush plus the cleanup, I just brush it on with a small paint brush and I get great results and the cleanup is a breeze. Plus, it ensures that I get into all the crevices and the undercuts.

[Mindenbrush]

I used to use Humbrol Enamel Matt Black but less and less hobby stores were stocking it here and the last tinlet was unusable - more like grease than paint.

A trip to Home Depot provided me with a 237ML can of Tremclad Flat Black Rust Paint for double the price but 6 times the quantity.
As with the old BlackFriars varnish, the tin is left upside down to allow the pigment to settle on the underside of the lid.

Open, scoop out some and thin with Testor's thinner - I find that regular store bought paint thinner gives the flat black a slight satin finish - and brush on.

[Golgotha]

For hard plastic and metal usually prime and undercoat with a light grey spray paint from a can. Covers well highlighting harder-to-see areas and details and then takes paint well - ticks all the boxes.

[Digits]

Halfords Matt black spray auto paint for everything.