Because a tarp is generally quite large, you will need to dye it in a large wash tub or a bathtub instead of your washing machine, then wash it in an industrial-sized washing machine before and after dyeing to ensure that it is properly cleaned. Any fiber-reactive dye will work on canvas and will produce better, longer-lasting results than clothing dyes like RIT.
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Wash the canvas tarp thoroughly.
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Fill the tub or bath with at least 20 gallons of warm water, pouring in the non-iodized salt at the same time so that it dissolves completely.
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Dissolve the dye in about 2 cups of warm water (or according to manufacturer's instructions), then pour the dye into the tub of water. Stir slowly to make sure the dye is completely dissolved throughout the water.
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Place the canvas tarp in the dye water, and stir it every few minutes for at least 20 minutes.
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Dissolve the soda ash fixer into about 2 cups of warm water. Over the course of 15 minutes, add the dissolved soda ash to the dye water, stirring constantly. Do not allow the soda ash mixture to touch the tarp directly—move it aside with the stick or paddle.
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Continue stirring the dye bath every three to four minutes for the next 30 minutes (light colors) to one hour (dark colors or red).
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Drain the water and rinse any excess dye from the tarp. Wash the tarp in hot water to remove the rest of the excess dye.