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They may have recommended a product suitable for cleaning the canvas furniture. A lot of canvas furniture has special waterproofing features which must be considered when using a commercial cleaning product. When selecting a product, check that it doesn't affect any waterproofing placed on the canvas. Some suggested options from a sweep of various online forums include: Barkeeper's Friend, LAs Totally Awesome and Spot Shot Upholstery Stain Remover. However, such products tend to be very strong and may cause new stains. If you do use them, always test a small patch first, to see if the product stains or removes waterproofing properties.
Check the manufacturer's instructions. Ask for a recommendation at a store that sells canvas furniture.