I also sweep the untouched surface edge with the back of my glove making sure to sweep any material that is on the untouched asphalt back into the worked area. I also only pinch the two edges together running my walk behind roller mostly on the outer surface. This needs to be done as soon as possible. Pinching the edges together is the most crucial part. When your first score the asphalt, I like to get a sharp angled edge around the area to be worked using an asphalt rake turned over (the flat edge). By angling the edges, they seem to weld together better. You also might not be going deep enough around the edges.
Hope this helps!
