From my experience of using several galv yards over the years I'd say that the quality is achieved through a good dip spec. If you specify to them them what you want yo will get a good finish and no warping, with a bulkhead for example the key is to specify that it is air dried and not cold water quenched etc.
Also spec that the galv will be your finish therefore they will know to keep skimming the top to keep the dip clean as it comes out. If they think it is just a component you are going to sand and paint, they take less care.
