For the same reasons Galvanising wouldn't work as you would be adding to the size of all the parts, and galvanising is done like plating isnt it? ie you have no choice where the metal coating goes, therefore it would plate the insides of all screw holes, all screw threads etc etc and i'de be suprised if you could even rebuild it.Some markers have such low tollerances and are made to fit exactly so that taking the marker apart and painting the wrong bit too thick will effect how well it will fit back together and how the componets sit together that it may/will bring more problems than its worth.