Today I found canvas at a local fabric store. But no one who works there can tell me what the ounce per yard is or if it is even a #4 or #10 duck canvas. It is all on sale and they seem to have a variety of varying thicknesses. the brand manme is "Oak Hill" and "River Hill" So is there anyway to visually tell if it is thick enough? And will the colored ones affect the waterproofing of a kayak? Any input is appreciated.

Rick