The fiber cement siding deteriorates. The top layer has weakened in bits, and flakes large and small hang on by tenuous connection. Some of these fall when you go over them with the scraper. Higher up, little pockmarks where the paint as fallen off, dappled spots like an Appaloosa. It is worse at the edges where the once straight edge is now jagged. It becomes like the map of a seacoast. The inner layers of the siding are softer, grainier. Rain will hasten the decline, forestalled a bit by the new layer of paint. Brush strokes — the gray bits don’t take all the paint at once.
You must brush again.