It's time to repaint the trim in my house and my question is, it was painted with oil base paint and I want to repaint it with latex, what preparation do I need to do so it will stick?
For best results, wash the trim with a mild TSP solution, rinse well. Prime with a good quality primer such as Zinsser Bull's Eye 123. Finish with the top coat of your choice.
I'd repaint it with oil, it's a lot more durable and is going to be a totally compatible finish. Just a little cleaning and light sanding, then paint away. But that's just me and I'm good with oil paints which many people no longer are as latex has taken over due only to ease of use and low VOC's. It is not as good a finish on trim and probably never will be.
Kind of agree with mastercarpenty. Since it is interior trim (up close and personal), no latex will be as durable, or cleanable, as an oil. It also covers better with a thin coat, and tends to 'layout' better and give a more uniform look. Latex is just 'that', latex, a flexible coating. Flexible means it stays soft, and thus picks up hand prints and grime easier. And being a 'softer' coating, it just doesn't clean as easy as a 'hard' oil finish.
You would probably have to get an oil at a paint store, as it is getting hard to find in the box stores. Don't get more than a quart. That will do a lot of trim.
Outside is a different story. Latex (acrylic) is much more durable in the elements.
If you are determined to switch to latex, prep it as others have suggested....it is critical, as it can easily start to peel around any 'touching' areas if you don't. Wash it, light sand it, then prime it with a primer that specifies such use. Myself, I would clean it, then use an 'oil base' primer that will give me a flat finish, then apply my latex. The 'oil' will give you the bond, and the flat will give you a finish that the latex can grab.
There is now an alternative to latex/acrylic paints for woodwork. Sherwin Williams, Ben Moore and Behr all have water based Alkyd paints. These give the advantages of oil paints with the convenience of water clean up. I have been selling the Behr Alkyd for a couple years now and have had nothing but positive feedback. It was developed for the California market which has just about banned oil products.
One of the advantages is that it can go directly over a sound existing coating without a primer. It has excellent adhesion. It even feels like oil paint as it dries. It feels sticky, unlike latex paints. The good news is that you can go to the sink and wash it off with soap and warm water.
Like oil paint, its recoat time is several hours. Whereas the surface will feel dry within a couple hours, internally it is still drying through the polymerization process, as does a true oil. It does not have the same work time as a true oil. One is well advised to work deliberately, and not overly brush the surface. You will see initial brush strokes, but they will level out.
I have been using this in the recoating of my own woodwork, which was originally sprayed with latex paint by the builder. The Behr Alkyd levels itself so well that I can hardly tell what was sprayed and what I brushed. Of course, the Alkyd can also be sprayed, but airless spray equipment is advised.