Having had problems getting Douglas castings welded in the past, I sought advice from an aerospace welder. It was to mechanically clean (wire brush) the parts first. Then set up the welding set so that the current is too low to weld, and the arc bias set to maximum cleaning. Then go over the joint so that the arc cleans the area without actually welding, wire brush again and repeat the operation. Make sure all parts are correctly positioned, set the welding machine to the usual settings and weld up as normal.
Hope this helps,
Regards,
Eddie.