Scars are a mark left on your skin after injury and stretch marks are small tears in the skin - this skin doesn't rejuvenate at the same rate as clear skin. Our studies on scar tissue lead us to not recommend application on scars or stretch marks, as the ink could get trapped in the scar or stretch mark tissue.
As well, scar tissue tends to be thicker. Because our ink needs to seep into the first layer of skin to develop, thicker skin can act as a barrier, preventing proper development.
If you do apply over a scar and ink becomes trapped in the scar tissue, it is biodegradable, so it should fade over time.