Often my back hurts, even after all these years, becuse my posture is awful; too much time spent in front of computers! But I try not to think about that and do my best.
What I do is the first 5 minutes or so, is hongsau, but on the out breath, tighten/flex lower back muscles, the outside ones and the inside if possible, then totally relax them on the in breath. Just like PY mentions you can heal by focusing on the area and tighten the muscle. It takes a lot of time like anything else, but eventually it has helped, over time it can only strengthen at least.