I just had new windows installed and want to paint the old existing interior wood casings. Do I have to sand them first? Apply primer or just paint them with a semi-gloss paint? They are dark brown and I want to paint them white. Also have two doors with unpainted wood casing. Same process? Thanks.
If they are a dark brown, I'm assuming they are at least stained and possibly poly'd (or other clear coat) wood
In that case, (and if they are bare wood or the coating, if any, is unknown), the best course is to sand, dust, prime, coat (premium paint, preferable an acrylic enamel), re-coat
Please be advised that based on your specific application, addition of a few mils of coating like this on the inside on the window casing where the windows themselves run/slide, can affect how easy they slide
...usually not a terrible issue, just if they are real tight now, it may make them a bit "sticky" (in a sliding sense)