I believe you put the negative of the edge you are looking for inside the form.
Something like this:
http://www.expressions-ltd.com/Concrete_Edge_1_5_Inch_Chiseled_Rock_p/crete-edge-1_5in-chisel.htm
The above are rubber and can take the shape of your mold and can be reused.
Another option would be to carve the look into 2 or 3 pieces of wood for that leading of your form. If done in multiple pieces it won't lock onto the part during removal.
Greenbikemike, nice job, getting ready to do the same style of project as part of a basement remodel on a straight run of wall vs. a corner. Using a similar river rock and wonder how you handled the edge transitions to the drywall with the irregular edge of the river rock.
I believe you put the negative of the edge you are looking for inside the form.
Something like this:
http://www.expressions-ltd.com/Concrete_Edge_1_5_Inch_Chiseled_Rock_p/crete-edge-1_5in-chisel.htm
The above are rubber and can take the shape of your mold and can be reused.
Another option would be to carve the look into 2 or 3 pieces of wood for that leading of your form. If done in multiple pieces it won't lock onto the part during removal.
I'm sure you can put something together given time and ingenuity. The product recommended simply removes those factors and makes it super convenient. I mean you could use clay in the mold corner. Just remember that if the finished product has a concave 'break' that you have to carve that as convex. You'd have to be fairly skillful.
You can get some RTV and make the product yourself a short section at a time and join it with clay or wax. As I said, the company that makes that simply helps move the project along.
