If we add too little buffer, there will not be enough buffer molecules to interact with the acid and the pH will not get into the buffer range. If we add too much nothing drastic will happen if the buffer is compatible with paper integrity. I personally don't use much Wei T'o and still have a can from years ago. This is because of expense rather than fear. It is simply an organic delivery system for magnesium carbonate that is otherwise difficult to apply.
Magnesium carbonate is the buffer in Wei T'o.
I am not recommending people use Wei T'o, but it is there for some tough problems. It simply measures the hydrogen ion concentration. We should be careful about the terms we use, though, because that can cause inadvertant confusion.