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Get out a small bucket and pour 2 tablespoons (30 ml) of gentle dishwashing soap into it. Use a dishwashing soap that will cut through grease and grime. Pour 1 gallon (3.8 liters) of hot water into the bucket. Stir the mixture until it's soapy and combined. Dip a sponge or soft cloth into the soapy cleaning solution. Wipe the solution along the bottom and sides of the enamel tub. Gently scrub the tub to loosen any grime or soap deposits. Avoid using sponges that have an abrasive side, which could damage the enamel. Fill a clean bucket with clean water and pour it over the soapy tub to rinse it. You may need to fill the bucket a few times to remove all of the soapy residue. Wipe the tub dry.  You could also turn on the shower and direct the nozzle to rinse away the soap. If your shower head is a handheld model, this is the easiest way to rinse the bathtub. Since you'll be drying the tub immediately, you can use any temperature of water to rinse it. Remember to clean your tub at least once a week or more, if you use it more frequently. If you keep the enamel tub clean on a regular basis, it's less likely to develop stains or limescale buildup.
Mix hot water and liquid dishwashing soap. Rub the cleaning solution over the entire tub. Rinse the tub with clean water. Clean the enamel tub every week.