[sick472]

All good advice so far and ranging from complete re-do to not so much work. My suggestion below is likely the least amount of work and should suffice knowing that your truck is a fair weather driver...

I would consider cleaning them with a solvent, maybe lacquer thinner, not alot, just a dampened corner of a rag. Work it into the cracks well and wipe clean and dry well with a light amount of compressed air. Certainly try this somewhere that is hidden to prove that you will not be messing up the paint with your choice of thinner. Don't use mineral spirits (too oily for touch up paint IMO), but consider lacquer, but alcohol based wax and oil removers would work well and be less chancy.

For the areas that are chipped and show the darker undercoating, I'd clean with the solvent and touch up with a small paint brush with as close to a color match as you can. This may take a few applications to cover the dark undercoating and fill in the cracks....then,

Use a medium to light buffing compound on any of the brown stained areas to brighten them up. I am talking about a t-shirt rag wrapped around the tip of a finger and a dab of compound to clean off the stain. Then work wax into the cracks to help preserve it. Wax will go along way in fighting the growth of rust in those cracks. I'd use the ole'school wax like Turtle, not the spray on quick waxes or the runny squeeze bottle stuff. You need something rather thick and substantial that will stay in the cracks and not evaporate or eventually run out in the heat of the sun. re-apply the wax in those areas after every wash or after every few rain events.

I think this has potential to keep it looking good for as long as possible, but if the blemishes are more than you care to accept, a redo will certainly buy you many more years. Old vehicle drip rails are just going to do this and our trucks are more prone than most for some reason.