Well I don't know about not damaging the material, but bleach and water sprayed on for several minutes then washed with a mild detergent might work...somebody told me peroxide and baking soda made a good cleaner...50/50 mix...but you can buy tarps cheap, surplus material....

http://www.aasurplus.com/Products/148-tarp-vinyl-gi-rubberized.aspx

it's a fitted tarp that would have to be custom made.

Then I would follow the suppliers recommendation on cleaning and conditioning....

Here's one at random....

"

How to Re-treat a Canvas Tarp?

A canvas tarp can be retreated with chemicals designed to treat canvas tarps. These canvas tarp treatment chemicals, such as Canvak are available in our tarps accessories section.

How to Wash or Clean Canvas Tarp?

A canvas tarp can be washed with soapy water or detergents specifically designed for cleaning canvas tarps. Do not wash your canvas tarps with your laundry washer or dry canvas tarps with your laundry dryer. Canvas tarps have wax and dye, which may stain your laundry washer and dryer."

http://www.mytarp.com/canvas-tarps.aspx

http://www.coversuperstore.com/starbrite-canvas-cleaner-16-fl-oz

this is a custom made boat cover that fits a 15' boat and weights approx. 100#s.

Not boat tarps.........mine cost 2 g's.

Soap and cold water (hot water will cause the tarp to shrink). Bleach or peroxide will remove dye. For mould use baking soda scrub.

If material hasn't degraded there are many waterproofing agents available from marine supply stores.

My favorite cleaner is tri-sodium-phosphate. It removes mold and mildew without the damage bleach can do. Use gloves, it's harsh on skin.

I expect you can follow up with Skotch-Guard or Thompson's Water-Seal if you can't find a special canvas sealer. Check upholstery and convertible top shops for good sealers.

thanks. g.a.

I take the tarp for for my boat to an oriental rug cleaners for cleaning. They are so good and so inexpensive that it is not worth my time and efforts to attempt to do it again. When I did it myself years ago I wasn't able to do nearly as good a job as they do. An old guy with a Friendship sloop put me onto this. He was so tight with his money I knew it had to be worth the few dollars it costs me.

I have a local sail maker clean the sail cover when he does my sails every few years. Again he does a much better job than I could and at a very reasonable rate.

Look into doing this way as an alternate. It could be much cheaper and better than you would expect.

Good Luck, Old Salt

Good idea, as long as you tell them to not use any starch.

A cheaper alternative to the commercial products, but has basically the same ingredients is Muriatic Acid(Brick acid). Dilute it to a 10% solution.Check label for the concentration of the particular brand you buy, because strength varies among brands.

Observe all safety precautions when using,and never add water to the acid when diluting;add the acid to the water to avoid a sometimes violent reaction(depends on Ph of local water, strength of acid,etc,but this is a standard procedure when diluting acids of all types).

This will clean effectively,but remember to rinse thoroughly after using.

Happy sailing.

Chlorine Dioxide cleans and kills mold and pathogens. This is what they used in New Orleans to get rid of black mold in water damaged homes. They came out sparkling also local neighbours hung their damaged clothing in the home to be gassed and it cleaned up all their mold and mildew issues. You can try some H2O2 3 % may be too weak so maybe 9% used in hair bleaching will do the job. 33% is food grade but use care and rubber gloves in handling it. I have destroyed cotton fabric with regular bleach when solution was too strong.

We use a product called "Wet and Forget". It's available at most hardware stores. It comes concentrated and is diluted 5:1 with water and sprayed on the affected surface. It doesn't work instantly, but after a few days the mildew, fungus, etc. just fade away and don't return.

I like "Quik & Brite" to clean such things. Never damages anything, but does a wonderful cleaning job. Been using it for probably 15 years or more....

The cover will probably need reproofing to make it water resistant again in some cases, tents and the like....

I used Mold Control by Concrobrium to clean my boat cover once and I also used it in the basement on plywood panel. It kills mold and supposedly keeps it from growing back. Improved the smell in the basement. Did see a little color difference but then I'm not sure that it was caused by being cleaner or because of some action on the stain in the plywood. Test first for color fastness on your canvas especially if it's a dark color.

I used the Wet & Forget on outside foundation, vinyl fence and roof. Works great there but never used it on canvas. Worth a try. It's probably a copper sulfate mix (from blue color of mixture) since it kills roof fungus real well and they always say to use either copper or zinc strips on your roof to remove the black stain you often get.

A great natural and cheap method is white vinegar followed by a quick brush scrub with baking soda. Rinse and hang up out in a bright sunlight. Vinegar is a great cleaning and disinfecting agent. A tablespoon or two of baking soda followed by a cup of vinegar freshens up and cleans a sink drain better than some expensive drain cleaners.