I have built my first two SOF on stuff from local fibric store. It is cheaper and ,as a student, don't know any better. Both held up pretty well until one fell off the roof from a moving car -- well, we don't expect that it would survive a fall at 60 miles per hour, do we? The other boat was given away and don't know how or where it is.
When I asked the fibric store for cotton cavas to build a yak, the lady gave me this horrify look on her face. She went to talk to the manager and come back later. She refused to sell anything to me that day. I learnt my lesson, came back a few days later and bought the heaviest canvas from someone else.
If you approach any salesperson, be aware that he / she may not be wanted to be responsible for drowning links to his / her conscience. He /she may not know what you need for your project and don't have the confidence to make recommandation. Saleperson is only required to tell French lace from Italian. How often you expect someone to ask him / her for boat material. He / her may nor even know boat can be and has been built from cotton Canvas!

: 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