RCS.......
The fenders were painted and then attached to the bed. The fender attaching bolts you see in the bed were painted uninstall. Used a sheet of Alu. with holes drilled and taped bolt thread size to hold them for painting.
The washer may be a good idea, would test fit bed/fender with them to see how it works. Redueing a stepper is a lot of work, however, it is worth it in the end.
Did not use seam sealer between bed and fenders. Would test fit fenders to bed before painting, to make any adjustments needed.
Hope the helps, the write-up earlier in this thread by Jeff really helped me see all the steps needed to prep for painting.
Les