This 3 inch stain has been here before we moved in. It appears the that something has worn away the laminate off and is exposing the particle board. There is no dent or depression, but I can feel the edge slightly.
How do I repair or remove this stain, without replacing the entire countertop?
Since it looks like it might be a rust stain, you could try white vinegar. Iron oxide plus acetic acid yields iron acetate, which is water soluble. But that's just something I might attempt; no promises that it will work, or indeed that it won't make matters worse.
I don't think paint will match the existing surface, so if you want it completely invisible you'd be looking at painting the whole thing, which then becomes a maintenance hassle.
If space permits, I favor crip659's idea of finding something attractive to sit on top of it and forgetting about it. If not... replacing the whole countertop, or the whole vanity, is a lot of work but might not be unreasonable if this seriously bothers you.
Plastic laminate is generally not considered repairable. It's plastic, after all, so sanding, painting, and other destructive repairs fundamentally change the nature of the surface. You'd be left with something much less durable.
Judging by the embedded grime, this top is due for replacement anyway. Fortunately it's not terribly difficult. Feel free to post a new question about how to do that.
