This is from the manufacturer of Simtex, Spradling:
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Simtex General Care and Cleaning Guide
Step 1: For light soiling, a solution of 10% household liquid dishsoap in warm water, applied
with a soft damp cloth. Rinse with clean water and dry.
Step 2: For heavy soiling, dampen a soft white cloth with a one to one (1:1) solution of Fantastik® and water or Formula 409® and water. Rub gently and rinse with a water dampened cloth.
Step 3: For more difficult stains, dampen a soft white cloth with a solution of household bleach (10% bleach / 90% water). Rub gently and rinse with a water dampened cloth to remove bleach concentration. Do not use with alcohol based cleaning agents!
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There is no mention of using 303 products...not sure if they are safe for this material, but they might be. I have had good luck taking the white sunblock marks off the vinyl using the first step of dishsoap and water with light scrubbing. Also, sunblock marks on black anodized rails can be removed by a little Armor All (or similar) on a soft rag.