I need to repair a stair handrail that has come out of my plaster wall. One end of the rail was not connected to a stud, so has come lose. It is a nice railing, so I would prefer not to buy a new one or cut it to fit. So what I am wondering is, can I cut a hole in the wall and put additional support in to support my already existing handrail?
You can install one or more handrail brackets under the railing and line them up directly with the studs. You should be able to find them at any hardware store:
Alternatively, yes, you can open the wall and install a nailer (2x4 installed horizontally between the studs), but this will require some plaster or drywall patching and painting. I would fix the nailer in place with screws to minimize the vibration that may cause more damage to the surrounding drywall. To avoid opening the wall up too much, I'm guessing you'll end up going in at an angle from the front and bottom of the nailer into the stud. It would be better to go in from the opposite side of the stud, but that would result in a lot more wall to repair (you want any joints to be on a stud).
Echoing what BMitch said. Don't fuss w/ trying to make a brace - doing that properly requires opening up the wall anyway - at which point, hey, you found the studs...
So just find the studs and add more brackets. Personally I think a handrail should be supported with a bracket in EVERY stud it passes.