So mandalorian armors don't end up in not-mandalorian hands. I don't know really, I'm just guessing from the fact that beskar seems to be hard to come by and mando has his beskar unpainted. I just assumed you don't paint berskar but it could be anything really. Maybe IG-11 is made of beskar too, that's why he can withstand so many blasts... Or maybe it's some sort of special metal alloy that's not beskar but almost as resistant. There's many ways to retcon this and other plotholes really, maybe that's one of the reasons we love this show so much. The main central plot holds so well so far, these minor details become not unimportant, but even interesting to discuss and speculate upon while waiting for season 2