Hello omega community! I have the following problem, the wheel box at the rear left is repeatedly infested with rust. After a rusting in the wheel box and under the decorative strip (left) I removed the loose rust, then ground the rest with the wire brush and Schirgelpapier, covered the resulting holes with fiberglass mats and pinned with polyester resin (Presto repair box), then spliced with filler filler (Presto), sanded and varnished. Now after approx. 4 weeks k The brown plague reappears and everything blooms (see pictures).What can you do about it?