I had a valiant 490 and used that 2 part glue by polymarine to re stick a wear patch that kept peeling back . Despite following advice using plenty of acetone to key the tube i never realy got what felt like a good bond , and the patch peeled off again .
I then used the rest of the glue on a very tired old narwhale and was never happy with the bond on that iether , the material on that apeared similar to the valiant .
Lots of vigorous cleaning with acetone was the answer , maybe the surface of the old pvc had gone a bit hard or something
recently i finished up the glue repairing the bladder to a ringo and it worked but gave a high build .
I also used a tube single of single part pvc adhesive on the ringo from an old boat repair kit , it was clear and seemed to give a better bond by melting into the pvc rather than sticking on top of it . Not sure where that came from it smelled like styrene cement and i felt it did a better job.