Thank you Carlos, jillanne, Jan and katherine.
I haven't noticed any discernible difference times with paper over canvas. Yes.a must to seal wood before putting paper over wood of any kind.
I had these A4 canvas lying around the house so I thought I might put them to good use.
After sealing with a graphite or pastel archival spray, I varnish with a spray matt. Then they are ready to hang. Much cheaper than glass and framing and you get to see the true beauty of the watercolour, which can be lost behind glass.