If I paint anything, it’s only the area under the points of the turnouts, but I ballast everything including the turnouts (but carefully there). Once ballasted and glued, it’s sealed. I have never had any issues of cork drying out - maybe if live in the desert.
I use a textured paint from a rattle can that is a good match for WS gray ballast. On the layout I am working on I only did it under the turnouts, so light ballasting there wouldn’t be so obvious. After I get the ballasting done on this one, if I get to a bigger layut I will decide whether to paint all roadbed or just the turnouts.
I paint the cork road bed then i ballast it including turn outs. That way if you miss anything when you ballast the track no big deal. But i lightly ballast the turnouts. what is 1 extra hour of painting