I'm sure Collonil products are good quality since Gucci gives them to their custumers. -Therefore I was surprised the stitching reacted this way..I use Collonil Waterstop on all of my bags and I'm delighted with the results, maybe you could give that a try.
Also, they've just released a new product called Carbon Pro. I've only tried it on one bag so far but the results were good and the protection is supposed to last much longer that Waterstop.