Are you using chrome bumpers or planning to paint or powdercoat them? If you plan to do chrome, keep in mind, you will need to use all metal to fill (lead). Not very easy to get perfectly straight, although do-able.
I've done smoothing on our bumpers and painted them. If you do plan to paint, it's not too bad. Just welding the holes closed, grind off the excess welds as smooth as you can then use bondo to smooth it.
Use black epoxy primer and a flex agent in the paint to strengthen the paint from rock chips and if you have black bumpers, the black epoxy primer will help mask the chip.
If you have any questions, I have quite a few pics.