I'm betting that the existing paint is a flat, and the vehicle, the water, of the new paint is penetrating the old finish and down to the substrate where it is vaporizing. Your going to have to put an oil base barrier between the coats of latex. You can do an entire oil prime, or do it in spots. Finish the first coat of latex. Never mess with blisters, leave them alone. Most will mostly go down as the paint dries. After the paint dries, sand out the remaining blisters, prime with oil, patch, oil prime, then touch up. The problem with this method is that once you start the second coat, you may activate areas that previously didn't blister, you'll have new problems, and you'll have to repeat the earlier steps. If the stairway can touch up and not look touched up, you're okay. The only certain way to go is the full oil prime. I've come across this once/twice before, inexplicable blisters. Never found why, just worked around it. These were in older houses, do you have one?