I find that rather than starting with a darker colour, a better method for avoiding lumps and bumps is to slowly build up really thin layers of white on top of a slightly off-white.
My suggestion would be undercoating with either Ulthuan Grey (for a cold white) or Screaming Skull (for a warm white), and then just layering three or four times with a diluted Ceramite White. By diluted, I mean you want the consistency of the paint to be similar in thickness to milk. All those are Citadel colours by the way.
As a former High Elf collector, I learned painting lots of white areas the hard way.
This method will take a little longer, maybe a total of 5 minutes per shield depending on the intricacy of your motif design.