Just fitted a new 41cm vanity unit in my downstairs cloakroom, the basin does not have an integrated overflow so I found a perforated waste (http://www.toolstation.com/shop/Plumbing/Traps+Waste/Flat+Grid+19+Hole+Brass+Basin+Waste+Without+Overflow+Unslotted/d20/sd2907/p83602) which I thought would solve the potential problem of overflow disasters. However to my great surprise when the tap is opened the basin fills with water and little or no water passes through the holes of the waste. To begin with I thought it must be a blocked waste outlet pipe but no. It seems that an air bubble forms under each hole which prevents the water passing through. Wiping a finger across the top of the grid clears the air pockets and allows flow. However in practice this solution to the overflow problem simply is not going to work unless I can find some way of preventing the air pockets forming. I guess this is something to do with the size of hole in relation to the surface tension of the water. Has anyone got an idea how I can overcome this problem??