Shouldn't there be a tag somewhere on the cover specifying what kind of fabric it's made out of? Assuming there is a tag (and it doesn't already include washing instructions), you can use other clothes you have (made out of the same fabric) for washing instructions.
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I found this link on the Maclaran website that suggests this is the case (and provides a fallback if you can't find it):
How do I wash the buggy seat fabric?
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Look for the fabric care instructions label on your seat. Most seats are designed to be hand washed in cold water, or sponge cleaned.
In my experience with the strollers we've gone through, if it feels like it has some sort of foam padding to help keep its shape, it'll be easier to get cleaner just by wiping it down - I find the washing machine doesn't work as well unless the fabric can "bunch up" (to rub against itself or other items in the wash).
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