You’re taking a friend on Monkey Face at Smith Rock. The plan is for you to lead the aid pitch, fix the rope, and have your partner ascend the fixed rope and clean the gear in standard aid climbing technique . . . but you remember that from the anchor, the rope goes over a rather sharply angled edge before drops to your partner.
Yes, thousands of people have done this pitch without any damage to the rope, but you'd still feel a little better if your rope was protected. This can be a pretty common situation in aid climbing, making a fixed rope anchor, or caving. Anytime you have a loaded rope over a rock edge, padding the edge or protecting the rope in some way is a fine idea. (Even more so if the rock has sharp crystals, like Joshua Tree granite.)
You can do this on the fly by putting a backpack or even a klemheist knot on the rope to protect it. But if you know you're going to have this situation, it's more secure to have a designated rope guard that you bring with you.
Here's an easy way to make one. You can use either heater hose from the car part store, which is convenient because you can buy it by the foot, or a section of cut up retired garden hose, if you have access to that. You need about 18 inches / 45 cm of hose.
Go to the car parts store, and buy a couple of feet of 1/2 inch heater hose. It shouldn’t cost more than about $1.50 per foot. (Some hoses used in cars are reinforced and cost more like for $5 a foot, don't buy this type.) The 1/2 inch hose works pretty well with about a 10 mm rope. You probably don't want to be using a fixed line that's much smaller than that anyway. You may want to actually bring your rope into the car parts store, and buy a hose with a diameter just slightly bigger than the rope you think you're going to use.
Carefully cut a slit the length of the hose with a utility knife or something similar.
Drill out a small hole an inch or so away from one end, and put in a keeper cord of shoelace, 2 mm cord, or some similar material. Remember, everything you take with you climbing needs to have a way to get clipped in.
To use, open the slit in the hose, insert your rope, and slide the hose so the middle of it is over the loaded edge. If you want to be extra cautious, you can wrap athletic tape around the hose, as well as a bit on the rope to secure it in place, Alternatively, you can put a prusik knot on the rope above the hose, and clip the prusik to the keeper cord to hold everything in the right spot.
Gates “green stripe” 1/2 inch heater hose
close up of Gates “green stripe” 1/2 inch heater hose
carefully slit it lengthwise . . .
insert your rope. Be sure and drill a small hole AND add a keeper cord to one end.
Rope all inside and ready to use.
To really make it secure, you can wrap the whole thing with athletic tape.