For all of the ribs that will make up the wing fuel tanks, Vans suggests scuffing all the flanges before straightening the flanges and fluting them, and before ever getting to the dimpling stage. It's a lot easier to scuff a flat flange than one with flutes and dimples.
I used 60 grit sand paper. Vans suggests using the coarsest grit, to give the Pro-Seal something to grab onto.
Using the same tool I used for the tail ribs, to make all the flanges 90 deg.
