The suction cups by Roth are a real quality product made for short term fixtures for camping, house and garden use. The metal parts are made of stainless steel, the rubber disc is of a material that is resistant to salt water. So this suction cup is ideal for sea lovers that stay on shore.
How much weight does the suction cup really hold?
That depends on the state of the rubber disc - it should be dry, clean and free of damages. In this condition placed on a glass pane, the suction cup carries 25 kg.
It also depends on the underground: the cleaner and smoother the area, the easier it is to create a vacuum and to keep it. You can test the underground yourself: pull your finger nails over a glass pane. You shouldn't be able to hear any sounds nor feel any unevenness.
Now do the same with the area you intend to place the suction cup: The more unevennesses and the more sounds you hear, the more the load is impacted.
Car paint in good condition usually is no problem at all. However, it can be a different story with foils, stickers, etc. You also need to be careful here that the suction cup does not peel of the stickers and foil when you remove it.
A few tips to keep your suction cup in best condition:
After use remove it and store it in the relaxed position, ideally in a clean, dark place.
Extreme temperatures, e.g. due to longterm direct solar radiation, can make the rubber age sooner.
The rubber disc should always be clean, free of oils and lubricants. Otherwise it may slide off...