Go to a local welding supply shop and get some "galvabar" you have to heat the metal up and melt it on and while it's still hot hit it with a wire brush. When we ran glavanized pipes on platforms and had to make an adjustment (cut, refit and weld) thats what we did to the weld after.
Gold Galvanizing Compound is your next best bet. That is what we sprayed on every piece of galvanized grating that we had to cut. It works well too. Make sure all the rust is off before you spray it though.
This is the galvabar...