I've had good luck painting Roundup on a fresh-cut stump, undiluted, right out of the jug, just like the label says. Dave sez within 20 minutes, but I thought the label sez 5 minutes, regardless, it's right on the label. I take the tip of my chainsaw and make a couple of shallow notches to help hold the liquid if the stump cut isn't level. I always buy the strongest formula of Roundup. There's several choices, and they can be misleading if you don't read carefully.
You can also use Crossbow, again, mixed as the label sez. I like it better than the Roundup, but can't say that it works better.