On Canvas Stretching...

The more important question to me is, why bother? With MDF giving you the size required, no tendency to stretch, good texture and archival quality, then WTF use canvas at all?

Lots of reasons - properly prepared, it's a pleasure to paint on, and very responsive to brush pressure. It's light and durable; and when a painting doesn't work out, you can dispose of it easily - or cut out a part you like, and restretch it on smaller stretchers.Paintings on canvas are very easy to transport (rolled or stretched). And on and on FWIW, I usually paint either on stretched canvas, or glued & gessoed oak plywood, I do like them both, but for different things. I used to use MDF, but found it less suitable in various ways than plywood. Plus I have yet to have any buyer react favourably to the aesthetic values of MDF...

Back to slick's question re. stretching canvas - I think a common problem that causes warping is stretching the canvas too tight before it's been wetted. I find stretching raw canvas just finger tight works fine - it'll tighten enough after sizing. (mileage varies, of course)

Because not everyone likes to paint on the same kind of surfaces.

Well, can you imagine the Pink Panther trying to cart off an 50# MDF panel in the middle of the night?

I hadn't thought of the security advantage!