IMO, the term "waterproofing" is very often made confusing by marketers. To me, a jacket that can keep you dry for a while is water repellant or water resistant. The only waterproof jackets are those that can keep water out even under hours of downpour.
Spray-on's / wash-in's can increase a jacket's water resistance. However, unless your jacket is waterproof to begin with (e.g. PU coating), after-market products won't make it waterproof. Too bad, cause otherwise, we can all be enjoying 3 oz. wp/b jackets! Obviously, this is not the case.
Here's an idea I have been toying with (but haven't actually tried): turn a wind jacket into a very highly water resistant jacket by using a combination of:
(1) Spray on water repellant treatment (or so-called waterproofing treatment in "marketer language").
(2) Coat the undersides of the jacket's shoulders and hood -- places where rain would hit the most -- with seam sealant or equivalent, to make those areas truly waterproof.
The end product is a very lightweight "hybrid" jacket that is (1) waterproof / non-breathable in strategic areas where rain hits the most, and (2) water resistant but very breathable in all other areas.