Sap on primer, what a bummer.
Step 1: clean all sap off with eucalyptus oil.
Step 2: Wash all affected parts with a heavy mix of sugar soap and water.
Step 3: lightly sand the car(600-800 DRY) to ensure there are no more spots.
Step 4: Spray a light coat of HI FILL,(Hi Fill has good sealing properties)
Step 5: Guide coat and sand off before applying colour coat.
I have sprayed about 20 cars, and I had (past tense) a gum tree that used to spit all over the cars. the above method has worked well a number of times.