Here's an update with some more photos to help get this job done right:
Photo 1: large areas will APPEAR not to need stripping and sanding down to the metal. But this is a deception, something like the fact that only the tip of an iceberg shows above water:
Other small patches may appear like this below, and one might be tempted to just wirebrush away the loose paint flakes and rust, and start painting:
This sanded down area reveals more of what's really going on under the paint. In reality, the constant bombardment of stones weakens the paint and lets air under it, causing a creeping spotty surface-rust over the entire area:
You might be tricked into thinking you've sanded deep enough, because two layers here look very similar, the layer with the main color stripped off, and the "down to the metal" surface. Here's a closeup. The key difference is the lighter area should be shiney silver and reflective:
Once you get down to the metal on both the rusty and non-rusty areas, you can spray on the "rust converter" treatment: This will darken the previously rusty areas (now a dull lightgray), and give a contrastier look like this below:
After successfully stripping down to metal and coating with rust converter, you're ready to paint a few layers (primer + color) and add the shock-absorbing ashphalt undercoating as a final dressing.
Now you can cruise over those dirt roads on the way to the cottage with confidence and peace of mind.