I would consider using used marine rubber hose of slightly > diamter that the rail, secured with standard stainless steel banding clips at the ends and ~0.5m o.c., cut to allow for uprights. Actually, hose of nominally the same size as th rail should be able to envelope the top horizontal pipe...
From what you have provided, it appears that the vertical and horizontals were fabricated from different batches of 'glavanaized' steel. one oxiidized, possibly pickled (light acid bath)for painting, the horizontal one could be 'fresh,' recently galvanized. The spec should control and should be...