That enamel coating is just there to prevent rust. It has [u]no[/u] effect on the magnetic circuit that produces the sound. To some extent, it may help by preventing loose magnets from vibrating in the bobbins. I suggest you get some clear fingernail polish, and dab a little bit on the uncovered area, to seal it up. Clear fingernail polish is one of those little 'trade secret' things. It works very well to repair dings on guitars, damaged speaker cones, and all manner of gear.