This trick comes from Sunstone Lab. After finishing a laminated socket, sand the edges and then rub the Sharpie over the edges. The pigment of the sharpie will absorb into the lamination and you can help the process along by rubbing thinner on the edges. If you are using a PETG inner socket be careful of what thinner you use because it may cause spider cracking of the plastic. Check out the following video:
The Sharpie Trick: Make the edges of your Above the Knee or Below the Knee prosthetic socket look great.