If the fabric can be waxed then
Barbour's thornproof dressing is as good as any other like product........I don't know about Fjallraven's stuff but most brands of wax dressings use a similar formula, despite their claims of "originality'
Edit: I've just done a quick google for the Fjallraven Montt coat & in fact it's a polyester/ cotton mix with 65% polyester, so I don't think a wax dressing is a good idea as the coat is not waxed in the first place.......it has a hydratic fnish, what ever that is, so be very careful & do your homework before putting anything on it......it looks like a great coat, it would be a shame to ruin it.....An e-mail to Fjallraven asking their advice would be the best thing to do.