Getting an even grey surface for white pencil drawing

I’m trying to find a way to achieve an even/flat grey paper surface to draw on with white pencils.

I can only find grey paper up to 300gsm, which isn’t heavy enough for me. Therefore, I’ve tried building a grey wash watercolour (wet on dry) but it’s proven very hard to keep it even over a large surface area (almost A1).

What are some method I could use that would still allow me to use white pencils on top?