Do you have pics of the pin knots? Depending on their size you can generally get down to a ring around them pretty successfully with a scraper. With a drawknife you have to be careful to work around them so as not to rip a splinter out near them. If you have them near the edge of your limb, you have to be very careful not to scalp them. As fat as those rings are you have a nice margin of error. Leaving wood from a ring above your desired ring is usually not a good idea. You can get them done, it just takes time; as you have found out.