Why Old Paint Must Be Scraped

The ultimate reason for scraping is to make the painting job perfect by producing a high quality painted surface with excellent smoothness and brilliance.

Scraping results in longer duration of the new paint and avoids peeling off if the old paint is not scraped.

In historic properties, paint removal  through heavy scraping is not encouraged unless it is extremely necessary. In effect, only light scraping, hand sanding and cleaning are applied as part of regular maintenance.

In the case of wood, new paint sticks strongly to existing paint than a bare wood, on the practical point of view. If the old paint still adheres tightly to the wood in a uniform manner and the surface has been carefully prepared for repainting, then, there is no need for stripping the old paint. But if deep cracks are already visible in the surface and a wide blistering and peeling of old paint appears, then, a complete removal of the old paint must be done before repainting.