When you say heavy cotton, how heavy? If it's classed as upholstery fabric it will do seat covers, but may be a bit heavy for curtains, if curtain fabric, maybe a bit light for seats.
Having said all this, I once bought a bolt of cutain fabric and made curtains from it, then covered all the chairs in my restaurant with it as well. It did wear through after about a year, but then again, it saw a fair amount of use in that time and I did fix it to the chairs with staples as it was a rush job.
Hope this helps.:k