Hel with repairing loudspeaker finish

I have loudspeakers that have a black finish with a satin sheen on the back. Unfortunately, one of them is scratched. I want to restore the nice, even, satiny finish. What's the best way to do this? The factors I have to deal with are:

I want to avoid spraying if possible. The speaker is very large and heavy (350 lbs.), and I do not wish to move it out of the room. I'd like to finish it "in place".

The underlying surface is MDF. I do NOT want a "wood grain" appearance. I simply want an even satin sheen. The current finish is mostly intact.

I looked at latex paint, but it seems too thick and heavy. The finish seems very fine.
Perhaps I should thin out the paint, or use a different kind? What about staining? Would it adequately cover the scratches? Any help would be greatly appreciated.

Any chance of a photo or two of the offending damage?

Black marker pen. After that, it depends on what the actual finish is....my initial guess would be that it's not a paint finish but some form of vinyl or melamine surface foil applied to the MDF, which is pretty hard to spot finish in an undetectable manner.