I have a pair of knickers (ie, 3/4 length trousers) I use for bike commuting year round. Since it's the start of the rainy season on the east coast, I was wondering if a spray-on waterproofing product such as Nikwax spray-on would add water repellency to my non-DWR polyester/spandex knickers or will this treatment perform poorly on non-DWR materials?
TL;DR - It will help, but it's no magic bullet.
Spray on DWR like nikwax or granger's will help the water bead up for a while when applied to knit fabrics. But the garment will still wet out with any significant rainfall, at best they will keep you from getting wet in a slight drizzle.
Most of these products have specialized versions for different fabrics, nikwax has cotton-proof and versions for fleece garments, etc. I've no idea if these are just different labels or actual real differences in the formulation.
The one thing to be aware of is that using these products will affect the wicking ability of any fabric you apply them to. This is not a big deal for something like a jacket or overpants, but for layers next to the skin it is likely not worth the effort.
In my experience, the best use for these products is to revive the DWR of something that came with a DWR in the first place.