I have a 2003 top on my 2001 S and the same problem is occurring on my top. Sagging at back, which causes wind flapping at speed due to the top not being secured at back of car.
I tried the 3M tape as suggested and it did not hold the canvas. I am thinking of using Aquaseal instead and will report back in a few weeks.
I think I will remove the top and work on it in the garage on a stand or on a blanket. Not sure as I might need top to be tight.
I'm hearing that from others too, including a chap who fits them for Porsche dealerships. I'm going to try the tape as I've bought it and I'll see what happens, if it fails I'll look at the contact adhesive option
Hybrid Mode
