[dentist90]

^^ my experience as well. If there is a fine layer of dust the car it may look just fine when you cover it, but when the cover comes off after a rain the dust spots become visible as the pattern of the fabric. If the fabric is "breathable" to allow moisture out you know it will also allow moisture in. Maybe not actual drops of water, but vapour penetrates nonetheless. I use my cover for long-term summer storage of my winter car (weeks to months without removal) while the Porsche gets the garage. It does a good job keeping bird poop, sap, pollen, etc off the car, protects the interior from UV and heat and protects against hail and scratch damage. If you are trying to keep spots off the car from rain you really need to keep the rain from hitting the car: garage or carport. Waterproof tarp works, but you already know the downside to these.