Client insisting on cheap materials - what to do?

[Paddy Roxburgh]

if possible, i would be armed with a price for "good" ply, and a sample of typical b and q stuff to show the voids. !

This is very good advice in my opinion. When I have to discuss ply quality with a customer I always show them the cheap bit of ply I used for my woodrat wall with voids and overlapping plys and then the perfect looking edge of some buffalo board or other quality bit of ply, I don't think anyone has argued the point beyond this