Welcome, yes, you need to do full prep. Scrape away any loose, flaking paint. Then sand the entire surface with with 80 grit sandpaper, giving the areas that were exposed a little extra attention. Dust if off with an old paint brush. Prime any exposed wood and then finish with two coats. I like to light sand the primed areas, and after the first finish coat, with 150 or so grit prior to finishing. If you need to do any caulking, do it after primer.
J pretty much nailed it, your topcoat will only be as good as the prep you put into it. It is easy to spend more time on prep than it does to apply the paint but it is necessary for a finish that will look good and last.
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