rust in the pinch welds around the bottom of the door

[Island Medic]

I am repairing the rust in the bottom of the door. I am not going to be using patch panels. I will be hand making the patches and welding them in. I was thinking of pouring some rust mort into the bottom of the door and allow it to go in the pitch welds to convert the rust that is there. I have been reading about the phosphoric acid. Sounds like it has to be rinsed out after the application. Has anyone done this, or should I just cut open the seam and clean it out mechanically?