this wonderful wood has the asthetic apeal of many people in its natural state, but with paint on it, the knots leach a ever leaching chemical called tannin that is difficult to control.
You can tame this beast by doing this: thoroughly clean the area with a good chemical cleaner, allow plenty of time for drying out. Very important. Then this process is important, find a good primer and a good quality of paint and this cannot be found in a home center, go to a paint dealer.
Use your primer and apply two coats over the area, allowing plenty of time for drying. Humidity outside affects drying time considerably. When your satisfied, then apply two coats of good latex finish coat.
I like Zinzer BIN latex primer and a good finish coat. Please consider a satin or semigloss sheen.