Angle grinders or a sanding attachment for the powerful drill are suitable for thorough rust removal. Thorough grinding means: There must be no rust spots. Otherwise the rust will soon bloom again.
First, coat the area around the hole with repair resin. A brush ensures a rich and even application. The repair fleece is pressed into the resin bed. It must be well saturated: Apply resin several times.
For rust holes, cut fiberglass mats are the only means. Do not try, to close smaller holes with more spatula: It is guaranteed to fall out due to the movements of the body parts. Press down the repair mat well.
The bigger the repair area, the more bumps it will have. Therefore, you fill larger recesses or any cracks in the mat with the first layer of fiberglass filler. It immediately adheres to the material of the mat.
Lutz Ptaszynski from TÜV Nord for professional repairs:
■ Löcher in der Karosserie nicht einfach zuspachteln. Better to close the hole by welding metal sheets, so as not to change the deformation behavior of the car.
■ Setzen Sie Glasfasermatte und Spachtel zum Abdecken von Löchern nie an tragenden Teilen ein. Not on the floor of the car and not on doors, since these serve as flank protection.
■ Spachteln Sie keine Löcher in der Karosserie zu, which were provided by the designers, this includes, for example, water drainage holes on the underside of the doors.
■ Deformationen und Risse in Stoßfängern aus Kunststoff können Sie ohne weiteres mit Glasfasermatte und Spachtel wiederherstellen.
Small paint damage, from falling rocks, you don't work with a grinder straight away, but loosens the rust with a rust eraser. Then the sanded area is treated with rust protection and varnish.