Just clean water and a brush (along with some good old fashioned elbow grease) is the best option for cleaning a canvas tilt and removing mildew. You can also use a 'canvas cleaner' if needed and Nikwax to re-proof it after cleaning.
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Remember always keep a wet canvas hood under tension to stop it shrinking.
Leave it on, or clean and then fit asap when still wet.
I remember pressure washing one ages ago, don't do it, it takes the colour out and looks a right mess.
Just water and a scrubbing brush first and if needed a canvas cleaner, followed by more clean water and elbow grease.
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As above clean it off the vehicle with a stiff brush and lots of effort then refit immidiatly and allow to dry fully refitted. I have never used the Nikwax waterproofing solution but am in the process of making my own waterproofing using an old fashioned recipe off the forum here with beeswax and linseed oil that I think will give a product similar to what is used on a wax jacket.