Resin Exudation
Cause
Resin and sap residue is not unusual where knots are present in wood.
A change in temperature can cause the natural resin to be activated and bleed to the surface. The practice years ago was to extract the resin by heat, burning out resinous knots before painting, but today this practice rarely happens.
The hard wood knots can also contain tannic acid which if not sealed will lead to yellowing of light coloured oil based paints.
Remedy
Remove excessive resin with solvent such as white spirit.
For wood translucent finishes sand down and seal all knots with clear shellac (knotting) followed by a conventional alkyd system of primer, undercoat and gloss.
In some cases if the resin in the hard wood veins is excessive, a leafing aluminium based primer should be considered.
Note: While the shellac knotting will reduce the possibility of yellowing it will not stop the resin coming to the surface.
Where the presence of resin is extreme then the only action is to have the timber replaced.