Should be fine. The steel you made the cart from is just a low carbon hot rolled product. The bolts are probably ungraded or grade 2 equivalent steel with zinc plating. You could grind off the plating prior to welding, then paint, but it will likely be fine to use the bolts with the zinc plating as well. The plating is very thin and the weld quality should be adequate, just don't hover over top the weld and suck in all of the fumes :-)