Don't assume Rob, a lot of folks just don't know the full reach of the great "Help/How To" threads on here.
J123 - if you're looking for the exact vinyl for strict originality/Concours - then get the Porsche kit. Otherwise, 914Rubber, Sierra Madre, Stoddard's, Auto Atlanta & a few others sell decent vinyl which is close, but not the exact "pepple" texture.
As Rob said - follow the cleaning, prep & application procedures religiously for the 3M Hi-Temp version of the adhesive to both surfaces, but there is also a Wurth adhesive also worth looking at.
It seems like recently there are more reports coming in of sail vinyl failures/peeling/bubbling/delaminating - so I think that there may be some issues with changed formulas for environmental reasons which could be making them less reliable.
I had a similar problem now in 2017, with delaminating/peeling/curling of INTERIOR vinyl using 3M Hi-Temp applied in 2014 using proper procedures on my `88 Westy - after just 3 years. I know my process & it still peeled, while it took the original factory application 20+ years to start peeling. So something is amiss!
I only add this warning - not to discourage you - but to say to say that even if you follow the procedures, then you may still have issues in a few years. Keeping the car covered or using one of Perry Kiehl's (member) top area umbrella covers will help keep direct sun heat off of the roll bar vinyl - possibly helping out some with longevity of the application.
PS - re: arne & Garland's comments above - the secret may be "years ago" before a possible formula change, since I have spots in my Westy where I used 77 & 90 maybe 10+ years ago & those have not yet failed. Eventually anything will dry out & come loose, like the factory stuff is doing after about 20 years on my Westy.
Cheers!
Tom
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