Regardless of what sort of coating your barbell has, eventually it will start to develop surface rust. Some coatings do a better job than others, but eventually all of them fail if you drop the ball with maintenance. To get rid of the surface rust, I would recommend using a soft nylon bristle brush and some barbell cleaning oil (you could also use WD-40 or a specialty rust removing solution from your local hardware store). Avoid strong rust removing products like CLR as they can remove certain barbell coatings completely. These harsher agents will just expose more steel. This will end up making the rust return quicker than before. The cleaning process itself is rather easy, apply some oil onto the barbell and use the brush to get rid of the rust. I’d recommend first putting the oil onto a piece of paper towel and then dabbing the areas you want to clean. This way you will spilling the oil all over the floor. If the rust is more advanced, I would recommend getting a new bar. Deep pits of rust will not only compromise the structural integrity of the barbell but can also lead to infections. It is simply not worth trying to squeeze out a few extra months of use from a barbell which is destined for the scrapyard.