Exterior Staining Question

[Thomas Bank]

The other week on Ask This Old House there was a segment on refinishing an outdoor deck. Among the recommendations, Tommy advised using an oil based stain but suggesting the addition of a half pint of polyurethane to each gallon of stain. I have some WAGs on why he'd suggest that, but nothing concrete. Any ideas?

[Steve Schoene]

My guess is that the varnish is to be an additional binder for the pigment in the stain, preventing it from rubbing off when used on the deck. That wouldn't matter on siding where it isn't regularly touched. Too much and you end up with so much film that it behaves like the paint it has become, and flakes off over time that's why a half pint (1 cup) per gallon might work better than higher proportions of varnish to stain.

[Thomas Bank]

I hadn't really considered the binder angle as this was a deck stain that they were using in the first place. But I was thinking about the film angle and wondered if it would actually lessen the durability over time - especially suggesting a polyurethane.