How do you know if existing paint or stain is sound enough to recoat? There’s a very simple test:

The paint on this pine trim seems okay for recoating, but an adhesive strip is applied as a simple test.

When the strip is snapped off, the entire coating detaches at the paint/wood interface, so the existing system is unreliable. The dark striations on the back of the coating are actually a microscopic layer of wood; it appears that some mildew is also present.

Read the full instructions for this test in Contemporary Relationships between Wood & Finish.