I've got very noticeable self harm scars on both arms. They've been there for 17 years, and I've tried most things.
I was referred to plastic surgery about them on the NHS - they said they're nothing they can do given the extent of them. I think laser treatment only suitable for one or two scars, not loads over quite a big area. I also paid to see a private plastic surgeon (just in case the NHS one was fobbing me off) but he said the same.
I was referred to the red cross for camouflage treatment but it doesn't really hide them, some of my scars are a bit raised and unfortunately make up doesn't flatten them!
Unfortunately some people do judge, or stare, or make unkind comments, or sarcastic ones.
That's just the way it is, there's a lot of nasty or ignorant people in the world. And funnily enough even people you think would know better (GPs, nurses etc) 
Its easy to say - well at least you know who those people are and can avoid them. But when you work, or pick kids up from school or go anywhere where you have are forced to spend time with people you may not necessarily like on a regular basis, that's not actually true.
Also I'm quite a confident person now, obviously moved on from the self harm a very long time ago.
But I can be going about my day totally happy and then suddenly, when I'm totally not expecting it, or in the most inappropriate situation, someone comments or just asks a genuine question about my scars.
And it can totally throw me. Or embarrass me depending on the situation (business meeting, in front of my v young children etc).
And it basically reminds me of that horrible time, which I try very hard to forget about. So it can make me feel a bit upset, it can even spoil my day a little bit.
So that's why I don't show them. I usually wear long sleeves when I'm outside my house. And yes a few people have commented on the long sleeves too