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Goo Gone is a product made specifically for removing adhesives, or anything that is sticky or greasy in general. It can be purchased from a supermarket or online. A bottle will typically be around $5 USD or less.  There several Goo Gone products. The Original Goo Gone will work to remove the decal. You can also purchase it in a spray gel form, or buy the Goo Gone Sticker Lifter. They are slightly different products, but they work in the same way. You can also make a homemade Goo Gone by adding 1 part vegetable oil with 2 parts baking soda. Put the contents in a bowl or jar. Take the product and apply it to the decal with a rag or paper towel. If using a spray, spray it on and use a paper towel to spread the product over all of the decal. Use as much product as necessary to cover the entire decal. Once it is covered with Goo Gone, you should leave it on the decal for about 4 or 5 minutes. The decal should be loosened up at that point. Then, take a plastic ice scraper or similar tool and lift the decal from the bathtub. Start around the edges and work towards the center. Continue to use the tool until the decal has been completely removed. Use more Goo Gone if any sticky residue remains. Use hot, soapy water with a sponge or rag and wash the surface of the bathtub. This is to remove any Goo Gone residue. The adhesive residue should be gone at this point.
Purchase Goo Gone. Apply the product on the decal. Allow a few minutes for the product to sit. Wipe the surface.