you usually just plate the outside if it is cracked or deteriorating. I would just clean up the outside with a wire wheel and call it good. The problem with plating over the entire outside of the frame is that you create a void between the two pieces of metal where water can accumulate and cause corrosion. In this area, you could have accelerated failure of your frame.
But I am sure shawn will come on and completely disagree with me
I was welding,, chillin, drinkin a brew, then I here some popping or cracking coming from the metal I just welded. crack through the weld I just laid down, along the seem??? anyone..
Are you welding in stitches? An inch here, an inch there, so you distrubute the heat over the area more evenly. So you don't get one side hot, and mis-shaped, then weld the bottom in the wrong shape. When it cools the fish plate would be under stress. That is my idea, there may be other reasons.
What thinkness are you welding on? You may be pushing the limits of that 125 and not getting enough heat into your weld. What thickness is your steel you are welding? Do you have any photos so we can tell if you are getting hot enough?