I was given a car cover from a friend. He bought it for his Mercedes, but drives it everyday, and has never used it. He lost the instructions.

What do I need to know?
How long can I leave it on?
What do I do if/ when it rains?

The material appearsd to be polylropelene with needle punches all thru it.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags

You can keep it on forever in any weather.

First things first, never ever put one in a dryer. They are the best things invented since air and sliced bread. i used one for the fall and winter of last year and it was great, no leaves, pine needles or snow to clean off the car. Best part was going out after it did snow and watching my neighbors cleaning snow and ice off their cars and whalllaaaaaaa all i had to do was dust it off and peel back the cover and i was good to go. Best 20.00 i ever spent. If it rains i'd stick it in a wash basket in the trunk and put it back on the car, they dry pretty quickly.

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"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

Depends on what the weather. If you have freezing rain or hail, or the rain freezes on it overnight, the cover will freeze to the car and you'll never get it off until it warms up. I use one on the SLC but I take it off when I'm expecting those conditions and expect to drive it the next morning.