zar oil based exterior poly http://www.ugl.com/zarWoodFinishing/zarExteriorPolyurethane/zarExteriorPoly.phpIts not designed for SOF but its covered hundreds of yaks with very happy endings. Its about 25 dollars for a gallon. I recently covered two yaks with a gallon and still have extra. For the price, however, it makes more sence to buy a gallon and waste the extra. As far as one part poly, zar seems to be the most highly regarded. You could experiment with something slightly cheaper but why?
It needs to be oil based. The water based and quick drying will not soak the nylon completely.
It should be available locally. There is a dealer locator on the link posted above.
Scott