Painting Over Peeling Paint: You Must Remove it First
This is pretty basic, but here goes anyway. When you have a situation of peeling paint, you have to remove it before applying fresh paint, or your efforts will be ineffective. Scraping usually works best, but in these situations, it’s pretty clear that the wood has deteriorated due to the exposure to sun and rain, so paint isn’t going to hold well unless you sand first. A few passes with medium-grit sandpaper may be adequate to expose fresh wood, but without this sanding, the new paint isn’t going to bond.
– Bob Flexner