Hi,
Use brush tool
Use a basic round brush
Hardness to zero
color: 50% grey
Opacity: 10-24% (start with smallest)
Disable all curves except one. Start with the bottom layer.
Brush several times over the masks, where the hard edges are. The gray brush will reduce the contrast.
When ok, enable the next upper mask layer, and repeat.
Adjust color, opacity and hardness to fine-tune.
Regards
