Noni is absolutely right in her discussion comment; this text is non-native. "Solvant" is a spelling mistake, but "wettish drapery" is a hilarious mistake for "damp cloth" and could not have been written by a native English speaker.

This being so, "dash", which is at first sight nonsense, is presumably a wrong word-choice for the concept they wish to express. It is something you might do to the seat cover after washing it but should not do.

The only thing that occurs to me is the sense of "dash" as a verb which means to strike something forcefully against something. One way of drying clothes or fabrics which used to be used, especially for woollens, was to beat them against a surface, especially having previously covered them with a towel or something that would absorb the water beaten out of them. It is still a bit odd, but is the only way I can make sense of the word at all.

"DRYING A SHETLAND JUMPER
Squeeze out with the hands, put the jumper between the folds of a towel, and beat to extract the moisture, or the thicker wools may be put carefully through a wringer."
The Good Housekeeping Dictionary of Facts (1926)
http://lisburn.com/books/good-housekeeping/good-housekeeping...

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Note added at 1 hr (2011-11-13 10:02:13 GMT)
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Sorry, I didn't see Ty's answer before posting. But I think the beating idea is probably related specifically to drying, as an alternative to wringing: an excessively violent way of getting the moisture out of the fabric.

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Note added at 1 hr (2011-11-13 10:28:33 GMT)
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However, on reflection, it could refer to a washing method. In Spain, at least, people used to wet the clothes and then beat them against a surface to loosen the dirt.