I agree with you grasscarp, Buenchico's answer was spot on and deserves credit.
I have done this journey many times and it's cheap and easy, when you get off the plane head for the train station across the road and get the train to Sants, this is Barcelona's main station, where you get off and you now need the Macanet train on R1, you stay on the platform you are on and look for the train time on the electronic sign's above you, if it's late at night and you have missed the last train then another will take you as far as Mataro, it then parks up for the night, you then take a taxi from outside to Caldes d' Estrac, about 20 euro.