Cats are hanging out at your place because they find something attractive there. Whether it's food, companionship, or shelter it's pretty obvious they're going to seek it out. Remove food sources, cover garbage cans, and close up access to spaces under your home, porch. Feral cats often seek "wild" food too, so if they're catching rodents, that's good, but it's better if you control the pests yourself.
Cats seek out high perches from which they can survey the neighborhood. Whether it's a porch railing or the top of your car they feel safe from other animals that might harm them if they can get off the ground with a clear field of view. That means you are going to have little pawprints on your car unless it's garaged. In a worst-case scenario, they may be "marking" your car . . . a neighbor's cat used to routinely "spray" my RX's passenger-side windshield every few days to declare his ownership of the hood of the car. Yeah, I've had a little experience, not all of it effective - but here are a few ideas . . .
There are "shock mats" that use a 9v. battery to produce a mild shock (like a bug racket) when an animal steps on it. At $30 to $40, whether these are worth the expense is your call. Be careful that you turn it OFF before removing and entering your car. A water pistol is probably just as effective, provided you can spare the hours on watch to give the intruder a few squirts. Be careful not to squirt your car, because then YOU are the one wrecking the wash job.
Red pepper can discourage dogs, and most cats by messing with their sense of smell. Ammonia is a classic treatment (smells like another cat's urine/marker) as is tobacco (or nicotine solutions, sold as the organic bug killer "Black Leaf 40" in garden shops) that will tend to repel animals from a specific area. Don't put any of these compounds on your car, they may damage the finish, but treat the area around
it. Once an animal is discouraged several times from getting on your car, the training usually lasts for a few weeks.
Not everything works on every animal, so be patient and keep trying. Cat claws are made of keratin, the same material that's in your fingernails, so they shouldn't damage paint too deeply - however if they find a favorite scratching post (our feral cats prefer tires, if not my leg) they may divert their attentions there.
If all else fails, build a "cat tree" or "kitty condo" out of scraps of wood and carpeting. It doesn't have to be as elaborate as those you find in cat-centric homes or pet stores. Those are really expensive - but if you can build a birdhouse, you can DIY this easily. There are plenty of examples, even plans available online. It doesn't even have to be a fine work of cabinetry, those cats don't care . . . so long as it is secure, soft, and high.
The idea is to make it more inviting
than your car, pick a high spot that's still accessible, Cats can jump as much as 5 feet, so don't worry about building them an elaborate stairway - but make it easily accessible . . . . their curiosity will do the rest. Be sure it's in the shade and offers an unobstructed view of the surrounding area. Add a couple of toys, a handful of catnip growing in a small pot, and a little soft carpet can make it more attractive and comfortable. Placing it near your car or other venue your feral neighbors hang out can make it the preferred alternative to your car.