It was suggested that I spray or roll a waterproofing material on my T-111 siding before I paint. Will this make the new paint adhere good or will the paint tend to have less grip!
A waterproofing material is never good for more than a year. I have heard this recommendation all my life and have never heard it from a paint manufacturer.
My suggestion is, if painting, prime with a top quality primer then apply 2 coats of high quality finish. If staining, apply 2 coats now and 2 more in 18 months. After that you can generally count of staining every 24 months in your part of the world.