I used 1/16 inch acetal cut into 2.75 inch diameter disks under the rail stanchions. I sealed the acetal disks to the boat using lifecaulk silicone sealant. Then, i used Tefgel (available from Sailor's Solutions) to coat the stainless screws before installing them. I used the acetal because it is hard plastic that will not absorb any water, has zero conductance to avoid galvanic effects, and does not mush up into the bottom of the rail stanchion to block drainage. I used the Lifecaulk sealant because it is not an acetic acid cure silicone - acetic acid cure silicones create a long term corrosion problem on aluminum because they create a conductive and corrosive acetate salt over time. Lastly, I used the Tefgel based on advice from Broward Yachts and Pluckebaum Yachts. It really slows down the stainless screw -- aluminum corrosion setup.