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These are exterior tips for painting. One, always sand pretty much everything on the exterior on the siding if it possibly could be for the wood because that paint that old paint those old coatings have a tendency to come up and pop up during its life. So they fail. With that being said, there's a process of a product called peel stop out there that is a really good, it's a transparent glue that goes on after you sand the building to make sure the old gets stuck to the new and it doesn't pop right back up. And after you do that, you should fill all the gaps with some elastomeric caulking, because that will stop any kind of water intrusion getting in behind that wood to where it'll expand and contract and possibly pop that stuff back up again.
Then you prime all the areas and that way it's sealed up on the top coating and then it's ready for a couple applications of some really good paint and if that prep work isn't done thoroughly you're going to be back repainting that house in two years paying again for it. Make sure the prep work is done super thorough that way you have a long term result that will last a long time.