Pat down your rock wall with a rag or towel to loosen debris and dirt.
Put on a face mask that covers your mouth and nose.
Open your polyurethane paint can with a screwdriver. Stir the polyurethane with a paint stick.
Dip your paintbrush into the polyurethane and apply it to the wall. Repeat to cover all of the nooks and crannies of the wall with a coat of varnish.
Allow the wall to dry according to the manufacturer's instructions.
Apply a second coat of polyurethane to the rock wall.