From the picture that looks like the white liner I have in a few of my tanks. Because of the white liner reflects light so well, it is very easy to see any imperfections.
I have one that is perfect with no imperfections.
Another one I have had a few rust spots, but they all polished out with a whip/ wire spinning brush on a drill. The coating is thin enough that if any rust gets under the wire brush just cut the coating and cleaned the rust. The rest of the coating where it was firmly attached tot the base steel was not affected.
I have never tried to remove all the coating, there is no point. At least with the type of coating on my cylinders it is easy to inspect for rust (blistering) and it has been hard to remove the imperfection.
With some of the thicker coatings the situation may be different if you see any blistering due to rust.
I would not remove the coating unless there is a definite need due to rust under it.
BTW, the dark spots looks like someone tried already to remove the liner, but as long as it is smooth and well stuck to the steel, it is hard to remove.
Buceador con escafandra autónoma clásica.