Kill mould on canvas & treat it
1. Mould has formed as it is eating the fabric & dirt trapped in the material. The best way to get rid of it is kill it, and remove it, and prevent re-growth.
Killing:
NOTE THIS CAREFULLY: Add two dessertspoons of liquid bleach to 9 litres (or a nearly full bucket of water). That is little bit in a bucket of water. Too much & the bleach will damage the canvas.
WEAR GLOVES and...
Using a sponge and some 'Sard Wonder Soap', lightly wipe all the area, and sponge over. Don't make the mixture stronger as it will damage fabric & a little bit will kill the mould.
*NOTE- if you don't have Sard Wonder Soap use a hard water type soap used in laundrys.
NEXT:
Wait until the fabric is totally dry, and,
Prevention:
THEN, Buy yourself some copper sulphate crystals (Available at farm produce stores- Don't ask me why, but it is!) You need about 100Grams and they usually sell it by the Kilogram, so you don't need much. Last lot cost me about $1.00.
Mix two dessert spoon fulls (about 20 Grams) in 5 litres (or a half a bucket of water) mix until all the crystals are in the water & its pretty blue.
AGAIN - Don't add too much as making a stronger batch doesn't improve the recipe.
Wear gloves and...
Now you have two ways you can apply this mixture,
A. with a sponge, or,
B. put it in a hand sprayer and spray the mixture on.
You will need to wipe off the excess in both cases.
Don't inhale the mixture as its not good for you
This will provide protection for about three to four years if you're unlucky.
Its all available locally & is a cheap method of preventing mould.
As for applying a waterproofing compound on canvas that is also necessary.