Funnily enough the G60 Syncro does not have those flaps! I'm not quite sure if it's a Syncro chassis feature not to have them, or if standard 2WD G60s don't have them either! The area you've pictured above is exactly the area I'm doing, except I have had to actually replace all the way around the filler hole. The rust had progressed far more. What makes it a heck of a lot more complicated for me is that this shell has already been painted quite a number of years ago. The body shop basically completely bodged the "repair" of this section; they had simply tacked the repair section I gave them over the rust. I'm now redoing that work with insulation shoved into the cavity to stop welding heat affect the paint... hence the decision to lap-weld the piece in, welding at as low current as I can possibly go. For the underseal I'm using the stuff from VW straight over the Epoxy 121. I do like the original VW underseal. I haven't purchased any cavity wax yet. Some really good advice from GG above. I'm afraid in my case I'm just going to hope that liberal use of Epoxy 121 and cavity wax will make it last for another 20 years despite my likely improper coating / priming efforts.