There isn't any one way or easy way to do what you want. I would be very cautious of using strippers as I think you will find they will damage the undercoating and could cause an adhesion issue. If you must remove the paint layer I would suggest a wire wheel to gently remove the paint with as little damage to the shutz as possible. Of course realize you will remove some of the shutz/undercoat during this process. It is inevitable due to the texture. The original Wurth product is no longer available in formulation or color so take that into consideration. If you are going to attempt to match the OEM product you will need to custom formulate it.
I removed the OEM undercoating on my car chasing rust that mostly didn't exist. I replicated the Wurth shutz by tinting white Upol Gravatex. You can see my thread here if you like. :
http://forums.pelicanparts.com/paint-bodywork-discussion-forum/939550-shutz-color-match.html I did find that the Upol seems to be harder and less rubbery than the OEM shutz. It is also porous I guess is the best description. What I mean by that is when I attempted to top coat it there were little pin holes. In order to combat this, I had some Glasurit 22 tinted to match the undercoating color to seal the undercoat prior to top coating.
