If the surface of a Newtonian secondary mirror mount is already uneven...

If the surface of a Newtonian secondary mirror mount is already uneven, it can be reinforced with a suitable washer from a hardware store.

If the secondary mirror mount is uneven, the secondary mirror will twist slightly when you want to adjust (collimate) it precisely. This causes the adjustment screw to press unevenly against the surface. A washer is usually harder than the surface of the mount, and there is almost always enough space between the secondary mirror spider and the mount to accommodate a thin washer.

http://www.teleskop-spezialisten.de/shop/Newton/200mm-8-Zoll/Skywatcher-Explorer-200P-200-1000mm-Parabol-Newton-OTA::466.html