If its just seams you want to fill that have no big gaps I normally use correction fluid 'Tippex' ,sand down wet with fine paper ,it gives a great smooth finish .

For priming I always use an Automotive primer applied with the airbrush ,it sticks like brown stuff to a blanket and is sandable .

To remove the old paint I use either Mr Muscle oven cleaner or Fairy Power spray (not sure what they are called in the U.S )  But for both ,simply cover the model with it ,put it in a food bag or similar and seal the bag ,leave it for bout half an hour then wash off

Hope you get sorted

Chris.