Using Liquid Mask over water based paint

[Ashlandchris]

This might seem like an odd question, but I am wanting to paint a wavy Union Flag and that will obviously require some shading. It seems to me that the easiest way to do it would be to put the highlights and lowlights (black / white) down as a thin coat and then LM over the top, draw out the shape and then cut / paint as normal, knowing that as I do a section with a plain colour, the shading will already be in place. My question is whether this is the best way to do it or will the LM affect the thin coat of paint underneath and also, will the scalpel lines show (I don't think they will due to the "hard edges" of the paint)