Tattooing over scars

I'd imagine it could be a bit lumpy, the difference in texture might make the finished product look odd. Otherwise I can't see any reason there would be a problem with it.

Some scar tissue is expecially pain sensitive, which might be problem.

Ditto what's been said. Though, it depends on the scar. If it's just a surface scar, chances are it's not going to provide any issues. However, if it's a deeper scar (cut, bullet wound, etc.), it's going to be a bit more painful.

Most tattoo artists can work around it and if you're up for the pain, then they can typically make it look good.

You might be best off talking to a tattoo artist. That said, on the histologic level, scars are disorganized and normally for the most part bereft of the usual subcutaneous structures such that the skin adjoining the scar might be hypersensitive for the portion that is (sorry for the odd analogy) upstream with regard to skin innervation, wheras the skin that is downstream of the scar will usually be hyposensitive. Depending upon what part of the body and how big of a scar it could take weeks to never for normal skin sensation to return. The limiting factor though is probably going to be the basement membrane or rather the lack of it in a scar. My guess is that the scar isn't going to take up the ink very well, and it might be better to surround a scar with a design and then color that patch of skin to match the color of the scar, as opposed to trying to actually do anything to the scar itself. I've seen some really bad attempts at hiding a scar with a tattoos, but that doesn't necessarily mean that the opposite is impossible.

One point is that you will have to wait for the scar to fully heal. Angry red scars are a no. But if they fully healed then it should be ok.

My husband put his arm through a sliding glass window when he was ten. That and the reconstructive surgery afterward left him with some gnarly deep scars all over his forearm and hand. He got a tattoo on that arm specifically to cover up the scars on his forearm eight years later. You can't really see them unless you're looking for the scars and know they are there. They don't distort the tattoo at all, just make dimpled creases on the skin. It's black ink.

I had most of a large, but flat scar tattooed over several years ago. It did not hurt any more than the other areas of my arm that were tattooed (that is to say, it did not hurt at all, since I did not find the tattoo process painful, even on the scar), and other than the small portion that was not covered, you cannot see it anymore. Just my personal experience.

I'm a breast cancer survivor. I've known other survivors who had mastectomies w/o reconstruction and have gotten large tattoos instead. Of course, these are surgical scars that have been stitched up by pros, not random scars.