Waxing 65/35 polycotton would be very different to waxing 35/65 polycotton. If you used something like beeswax or paraffin as you might with a cotton canvas (instead of Nikwax or similar like you could use on synthetic fabric) it would need more frequent re-waxing and in the meantime the jacket is waxing whatever it comes in contact with. You'd be covering the whole garment and only a minority of the fibres would be absorbing the wax. Maybe not a fun project. Unlike nylon, polyester will hold very little water so it does have an advantage in rain in that way. A good canvas can stop you getting wet because of its weave and fibre content. The cotton fibres swell with water and lock to block rain from penetrating further, but you'll still see plenty of modern canvases with coatings that make that feature of the cotton weave redundant. Coatings reduce breathability which would already be compromised with the synthetic component, so you could get more dampness from the inside after waxing. Sometimes I carry an ultralight Japanese umbrella in the inside pocket of my truckers, it's not the least elegant solution.

Waxing 65/35 polycotton would be very different to waxing 35/65 polycotton. If you used something like beeswax or paraffin as you might with a cotton canvas (instead of Nikwax or similar like you could use on synthetic fabric) it would need more frequent re-waxing and in the meantime the jacket is waxing whatever it comes in contact with. You'd be covering the whole garment and only a minority of the fibres would be absorbing the wax. Maybe not a fun project. Unlike nylon, polyester will hold very little water so it does have an advantage in rain in that way. A good canvas can stop you getting wet because of its weave and fibre content. The cotton fibres swell with water and lock to block rain from penetrating further, but you'll still see plenty of modern canvases with coatings that make that feature of the cotton weave redundant. Coatings reduce breathability which would already be compromised with the synthetic component, so you could get more dampness from the inside after waxing. Sometimes I carry an ultralight Japanese umbrella in the inside pocket of my truckers, it's not the least elegant solution.

@Alex

Ironheart.co.uk product page calls this 65/35 polycotton and most of the retailers say it is 65 poly 35 cotton. That is different to how Bob Loblaw has put it in his question to you, so this might need to be cleared up. 65 cotton 35 poly is the far more common blend most people will know. Looking into Japanese ''Tetoron Cotton'' info it seems like it is 65% poly.