I bought a house with a cedar deck painted over. Painted over most likely means trying to hide wood rotting. Modern Cedar and redwood rarely cut from insect resistant core--to hard to find. Painting over mildew will only make it rot faster. Groves likely from them removing deeply "dry rotted" sections.

If salvageble: start by stripping off paint to see what is underneath. Need to kill any spores before repaint.(various deck restoration products generally contain some form of bleach that kills anything.

If after treating, looks salvageable, sand lightly where needed to eliminate groves, coat with transparent or semitransparent stain type product. Not an expert, but if want protection against UV and mildew which is built into quality deck stains. House paint a serious mistake--will peel and allow mildew to grow beneath it. Hopefully someone can add to this.