I've just spent the last two afternoons sewing fabric to reupholster an office chair. The fabric I chose because it was dark blue (it's for DS's room) and for upholstery has a pile kind of like chenille or jumbo cord. Anyway, given it needs a flame-retardant liner and given the edges are piped, I found myself stitching four layers of the outer and two layers of the liner (except at seams on the seat sides when another layer of fabric presented itself) and I can tell you I was at the absolute limit of what is possible on my domestic machine. It's a proper metal-chassis model (ie, the main inner frame is not nylon) so it has some poke but it struggled at times to penetrate the fabric and I broke three needles. Additionally, the thickness of the work relative to the rise of the foot offered some challenges.
So, the take-home messages are:
- you might need to add flame-retardant interlinings to your cushions (Terry's Fabrics do it by the metre)
- a domestic machine might struggle with the thickness and guage of canvas awnings
Finally, profabrics.co.uk specialise in outdoor fabrics including marine and related accessories, which might be worth you taking a look at.