Having seen something on youtube I've just put the second coat of water based) undercoat on my staircase spindles. 10 minutes to paint 20 spindles (10 minutes for 1 coat of paint)
I will need 3 coats of of undercoat applied fairly thinly to cover all the marks and chips (sanded back to bare wood) in my spindles. If using this technique I think the trick is to work really fast and not worry too much about covering any marks or chips etc. in the first coat. Also working fast and with a thin layer of paint there is little build up of paint on the crevices of fairly ornate turned spindles.