I have a Class 8 tractor trailer that the previous owner never removed the fifth wheel frame. When I removed it there is a great deal of rust between the top of the rail and the fifth wheel frame.

The top part of the frame rail is thinner in the center than at the edges because of this rust formation.

My question is can I use weld to build up the top section of the frame rail? I understand about the manufacturer saying not to weld on to the frame rail, but I'm really not even looking to put any kind of a weld with serious penetration. I'm just simply looking to build up the frame to where it was, grind it down a bit and let it be.

I am using a beta Mig 200 with a .035 wire. My thinking is that the MIG welder with a smaller wire will not create a lot of heat and perhaps I can just put the weld on without affecting the heat treated frame. The frame is thick enough to be safe as it is, but I'm concerned that if it gets any thinner if any more new rust forms, I could have a problem.