"Leon" wrote:

ALTERNATIBELY, Drill lots of holes to find your studs in a straight
line even with where the rail will go.

Cover all of those holes up with a 1x4 piece of wood to match the
hand rail, anchoring it to the studs you found. Mount your hand
rail to the board you just used to cover the holes.
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To expand on Leon's idea a bit, the object is to provide a
secure railing to a plastered wall with unknown stud locations.

Rather than use a 1x4, use a 1x8 oak board attached to the
wall with two rows of 1/4" molly boats on 8" centers.

The board provides the structure to mount the rail while also
spreading the rail load across a larger surface thus reducing
the load any specific molly carries.

Lew