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Seriously though, I've seen vibration alarms work - usually to keep pets off furniture, but the principle is the same. Turn it on and just leave it on the roof when you're not using the car.
You can also buy a shock mat - same principle, to keep pets off things - and drape it on the roof. when the cat jumps up, it gets a shock and skitters off.
I know the reviews aren't great, but I bought it because I was getting desperate. I would wake up everyday to find more paw prints all over the hood of my s2k, and after trying a bunch of homemade sprays, this was the only thing that worked.
Reviews on amazon are definitely mixed, but I found that as long as I added more powder every two or three weeks, it kept the cats away.
They have these motion sensor things to train cats to keep off furniture. Either it makes a noise cats hate, or sprays them with a shot of air or mist of water. After a couple times of this, they learn not to jump on the couch.
So I'm wondering if one of these things could work in your situation.
Of course, your neighborhood dictates some of this. If something that makes noise would piss off neighbors. If a device on your roof would be taken by someone if you left it overnight, etc.
I use citrus peel, which cats don't like on the Hood trunk & the top, to keep our cats & neighbourhood cats off my Mercury Capri soft top. I found navel orange peal the best. This was after trying a couple of cat repellents from the vet. None of them worked.
It did not work with my daughters can when she was here for a couple of months.
So you just eat an orange, and leave the peels on the roof? Its... Recycling!
I know cats don't like citrus, as there are citrus sprays to keep cats off furniture, etc. The actual peels seems like a better alternative instead of spraying stuff on your car, except it might attract Raccoons!
i tried the vinyl stuff with spikes on the bottom placed upside down on the soft top. it's kept cats off for 3 days now. wrapped some rocks with an old t-shirt to weigh it down.
Do you have a Daiso USA store anywhere near you? Here in California, it's a Japanese "Dollar Store" chain that sells interesting items. For $1.50, you can get this. It's a plastic mat with small blunt plastic spikes. Cats hate these. You can probably find these at other places too.