I am replacing some of the bathroom sink faucets in my house. The installation hardware includes 3 attaching pieces...the lug-nut, a thin metal washer (or flashing disk), and a rubber disk/foot. The instructions are unclear as to where to place the metal disk...either between the lug-nut and the underside of the sink (to act as a washer) or between the rubber foot and the top of the sink (directly in contact with the sink surface).

When I disassembled the old faucet, the metal disk/flashing was on the topside (between the rubber foot/seal and the sink). Is this the proper place for it and why is this done? It seems to me not a very good place (always wet) to place a piece of metal (even stainless) as it eventually rusts and you get stains on your sink top.

Put the washer between the nut and the underside of the sink.

Rubber washer goes on the top, to make a watertight seal between the tap and the sink. Metal washer goes underneath to act as a washer for the backnut.

Rubber washer goes on the top, to make a watertight seal between the tap and the sink. Metal washer goes underneath to act as a washer for the backnut.

Just to break the deadlock

And just for fun