Existing Condition
Older homes often have multiple layers of paint on trim. In this situation, it may take more preparation. First, you must scrape and sand the trim to leave a smoother finish. Then the trim should be dusted before painting.
When there are dogs in the home, there may be a case where the dog has left chew marks on the trim. In this case, the marks must be patched with wood filler and sanded.
On the other hand, if new molding has just been installed it will require time to fill all nail holes with puddy and caulk all seams. Raw wood requires primer before painting two additional coats of paint. In addition, changing a dark color trim to white will also take multiple coats to cover.