first things first make damned sure there isn't any power still hooked up to the old neon transformers,.,,,don't ask me how I came by this pearl of wisdom,.,,
From experience painting these old monsters what Si has suggested will work,...,...I have used aluminum flashing tape meant for duct work to patch the old neon holes left when you remove the porcelan receptacles,..if they are hard to unscrew you can try using channel lock pliers or whackem with a hammer and they break away easily,..just be careful not to bend the sheet metal in the process,...you really need to get all the old paint off first,..then etch it with a mild acid,... vinegar or stronger,... after we etch them good we apply the adhesive backed aluminum flashing tape to cover the holes then prime well with a primer for galvanized metal or black iron. then paint as Si suggests,..do not offer up a guarantee to the owner as these old monsters are notorious for paint peeling off prematurely,...don't ask me why they just do unless you use a zinc chromate self etching primer and that is probably not gonna be cost effective,...new skins would be a smarter approach if the owner is willing to spend the extra coin,..,.,last of all be forewarned hornets and wasps love these old boxes so be careful,...I've stepped off into nothing more than once rather than fight a cloud of these nasty pests before I realized I couldn't fly,...oh yea have fun while you are painting it,...