Remove any leaves or debris lying on top of horizontal markers before cleaning.
Clean dirt and grime with water and a nylon brush on large flat areas of the marker. Get in between lettering and decorations with a toothbrush to remove embedded grime.
Rinse marker with water and dry with a soft terrycloth towel.
Dampen a soft cloth with mineral oil and apply to the surface of the marker.
Scrub the marker with a nylon brush, followed up with a clean cloth to remove discoloration as you work across the marker. You may need to apply several applications of the mineral oil as you work.
Remove residual oil with a clean cloth, taking special care around the lettering.
Finish with a light application of WD-40 or car wax to put a sheen on the marker and protect it from the elements. Periodic re-application of a protectant with a soft cloth is recommended to keep the marker looking bright.