[dsraven]

it may be an idea to use some sort of home made spreader bar on he back side for the fender attachment bolts in order to avoid some of the waves that may come when tightening the bolts with no welting. that would spread the weght over a longer area and attempt to make both parts conform to the shape of the spreader bar. something with some heft to it would be best I suppose, instead of just the sheet metal from the fender flange.
the old fashioned sticky backed sandpaper discs used on the da sanders work great for using on the home made backer boards, just cut the paper to size and stick it on the broom handle or whatever you have made, be that a broom handle, office eraser or chunk of wood you have custom shaped. also, drawing the sander across at an angle and doing a cross hatch pattern will help to keep from digginhg a hole or pathway into the high build primer or body filler or whatever you are sanding