It is advised not to sand polypropylene, I know, I was skeptical at first as it goes against everything we're taught about bonding. But I've had multiple conversations over the 14 years I've been doing this with 3M to best approach this substrate.
The problem is the plastic itself, I go into a little detail about it in one of my videos linked below.
Fist you want to wash it down with washing up liquid, there are some contaminates alcohol won't touch so do this first, then wipe over with alcohol. Then you want to apply a plastic primer (I use 3M primer 94) After that you want a Glue designed for polypropylene, there aren't many about because it is incredibly hard to get a permanent bond.
Also, Alcantara does stretch, it stretches on the width. You want to manipulate you material into the curves then work it out from there.
