What do people use to clean canvas tents?
Depends what you’ve got on it.
If mud your best bet is to brush it off dry with a stiff brush, or hose it if you have to - although you will then need to dry the tent again thoroughly afterwards and may need to reproof it.
If it’s just a few specs of that black mould from being stored damp don’t worry about it, just get some sunshine on it to make sure it’s properly dry and won’t spread (UV kills the bacteria) and pack it carefully for the winter. Best to get it out and air again in the spring before you take it to site though.
Anything oil or grease based is probably best dealt with by a commercial cleaner and reproofing. Several of the canvas sellers have recommendations for cleaning companies.
I had to clean poster paint off my canvas tent once…
This is perhaps why the Freeborn festival of throwing paint about no longer goes outside their camp…
In any case, for that, I went outside the (pitched) tent in the rain and attacked it with a heavy scrubbing brush. If that had failed, I’d have been trying again later with the brush and some Cif.
As a general rule, a dustpan and brush is a good accessory to pack for any large tent, especially those with a fitted ground sheet.