I ended up priming it first and then repainting. That was also suggested by an employee of the paint department of a major home improvement store.
Here are a set of suggested steps.
1- Clean from the very top near shingles so that debris does not get into fresh paint.
2- Pressure wash from a distant using a wide spray. Pay attention to the underside of overlaps. Wait for it to dry.
3- Make sure you have time to do a segment before it rains. Plan to finish an hour before sunset to be able to see missed spots.
4- Use painters blue tape to protect areas not to be painted. Press firmly to prevent paint from going underneath the tape. Cover the floor. Use a 4 or 6 inch roller with 3/4" nap for flat part of surfaces. Need small brushes for corners, edges, and under overlaps.
I ended up wiping the brown stain on the board with a rag. A bright shiny yellow surface shows underneath those browned spots. Brighter than a fresh board. Same thing shows around the home. Mostly in spots that does not get much direct sun.
The reason for brown spots and bright yellow surface was not clear.