The slide rust is visually nasty, but check closely your hammer spring. MineOh, my slide is rusted from salt-water exposure while boating.
I tried it on an old 870 and I wasn't impressed. I really tried to over do the prep work and gave it plenty of cure time. It just didn't seem any better than spray paint, it wore of some parts during the first range trip.I've had good luck with Brownell's Alumahyde II paints.