I use a tub of SRA 135 and apply it with a small hobby brush. I use lead free solder and even with a rosin core it’s pretty necessary. I have a jar of Thermaltronics’ TMT-FP-1 as well but I wouldn’t buy it again, it smokes/vaporizes more than SRA 135 although that difference may be more negligible with leaded solder. TMT-FP-1 melts at a lower temperature so the tub is more manageable and it stays liquid on a board a bit better if you take a 15 minute break.