What sort of paint do I use for smooth surfaces inside? Wood. Already primed at the factory. Lots of detail so a brush is required, I think. Painting for appearance, the surfaces will not be used for anything other than appearance. What sort of paint do I use?
Try latex paint from a paint store or any place else that sells paint. Since it’s already primed all you need to do is pick out the color. Some stores sell smaller cans of paint for smaller surfaces. I’ve painted a lot of wood and walls.
A spray can could give you a nice finish and offer a lot of colors.
What are you going to put on the newly painted smooth surface? Personal experience has shown that latex paint on a bookshelf if a really bad idea. Books stick to it (esp ones with glossy or plastic covers.)
For mainly vertical surfaces I’d generally use latex. For mainly horizontal surfaces that are doing to see use, I’d use oil-based.
Another thing to investigate: there are paint additives (eg. Floetrol for latex paint, might be Penetrol for oil-based) that make the paint flow/settle nicer. I used them when painting new factory-primed interior doors that had a fair bit of texture and detail. Brush strokes would settle nicely and the doors look like they were sprayed. Of course, actually spraying works great too, but you generally have to do that outside.
Spend a few bucks for a good brush if it is going to show.
Make sure it is danged clean before you paint.
Carry on.
Clean thoroughly, dry and sand very lightly (scuff) prior to painting. 2 thin coats beat 1 thick coat. Allow a day or more to dry before using. Depending on the type of wood, you may or may not get a smooth surface. Poplar and pine will finish smoother than oak.
No one mentioned sheen. You probably want semi-gloss for painting wood.