Decent rain-repellant spray/treatment for darkcloth, backpack?

[Ivan J. Eberle]

Ha! Someone's made a video detailing my method, a breathable rain jacket. You can even loupe-focus using the arms of the jacket. Best part is that whenever you get caught out in a downpour, you've got a jacket and not some relatively useless darkcloth.
My jacket is a dark blue Columbia Titanium with waterproof zippers and Omnitech fabric. You could spend up to several hundred on another brand of jacket and not come away with anything better (quite possibly not as good). I seem to recall getting this one for half-off a few years ago at the end of the season, probably around/slightly under $100 USD.

[Drew Wiley]

I paid my dues with poncho-style raingear in the ole days - but they would serve as an
emergency shelter. Don't know much about that Omnitech fabric - a friend brought along
an experimental lightwt tent made of it this summer. Handled hard rain for several hours at
a time, but don't know what protracted rain would do to it.

[premortho]

My go to spray is outdoor Scotchgard. Works good for me.