You should be in good physical health with good eyesight for depth perception and colour recognition. You will need steady nerves and hands, good concentration and patience, mechanical aptitude, and manual dexterity.
You should have no respiratory difficulties, and no physical handicaps that would prevent you from lifting 50 pounds on a regular basis and 100 pounds occasionally. Since work is often outdoors, you should be prepared to face some unpleasant weather conditions.