Filing down a plate

There are a number of methods to block out unwanted portions of a hard pattern plate - physical removal as last resort. But it all depends on a number of factors, among them being the location of the offending line, size of the line, wether you are using an opaque or process ink, etc. Without seeing the line……..
However, if the line is small and is being covered by a second darker hue that the first, you might well produce a finished product that shows no unwanted line. If such not be the case, well, depending on the line’s locale, a frisket might serve. Should the line be in the center of the plate, you can cut a complex frisket; reduce the packing beneath the tympan at the point of impression; cut the tympan (shudder) to produce a crude frisket; or, as mentioned , reduce the actual line.
To do that requires a fine eye, steady hand, good lighting, magnifying glass, and proper tool. I’m thinking you don’t have access to a routing arm having small blade(?), so the next best tool would be a dental chisel. Or simply make your own fine-nibbed plane. If you are adept at carving, line removal will present no problem for you. But I must caution - one slip and the tool will skate across the plate thereby giving you an entirely new situation with which to cope.
I suggest a trial run to see if the line is masked by the second colour. And, ‘afore you approach the plate armed with scalpel - practice on a sacrificial piece of magnesium. It’s an interesting metal to touch up. Hope this helps.
Laurence

I’ve used a dremel successfully. Not sure if it was zinc or magnesium. Don’t know if sustained friction from this would be hot enough to get magnesium burning, though.

I wound up useing a dremel as well and it worked great