Pine has a lot of good qualities for chairmaking, it grows into large trees quickly, carves beautifully and has beautiful grain. However, when I was learning to paint chairs, it presented a great difficulty. The abundance of sticky sap that resides in pine does not like to be painted, and often the paint will chip off from the sap filled pores during the burnishing process, revealing little white patches in my black seats! This is problem really comes to light because I prefer to burnish hard to get a high sheen. For those content with a matte finish, this may be a non issue. Here is a photo of a nasty pitch pocket that I came across while making the prototype from my recent "Frankenchair". I chose this piece of pine knowing that it would be going into an experimental chair.
The problem comes from milk paints quick drying. This creates a dramatic surface tension that can form a weak bond to dirty or sappy wood. The photo above shows the sap pocket after heating with a heat gun to liquefy the surface sap. Next I rinse the seat with naptha twice (wear gloves!) The photo below shows the seat after rinsing.
Finally I stain the seat with a water based walnut stain, which raises the grain. As you can see, the stain covered the sappy area easily and gives me confidence that the paint will hold as well. I do use extra-bond in the first coat on the seat, sort of a belt and suspenders approach. Subsequent coats are plain milk paint. Following this process has yielded me trouble free results for years, regardless of the sap content of the seat.