Hello Agong, I might make one other suggestion to try here. Everyone has given you really sound suggestions. If you would like to try one other thing, you might try starting the arc in the center of the joint and then bring arc out to the side of the groove, step straight across to the other side of the groove and then push it back to the center again, then repeat the process over again building on the shelf that you have just formed, as mentioned, don't use a lot of upward gun angle and try to use a nice steady progression. Good luck and regards, aevald