I have done the same thing by putting the longest exposure as the bottom layer, then the next shorter exposure as the second layer and so on and then using layer masks. Just add each image as a layer in the longest exposure image without any mods.
If you were working in 2 layers only you could do this:
- Turn off the second (short exposure) layer which is on top of the long exposure layer
- Select the long exposure layer
- Use the lasso tool to select the blown out area
- Feather it by maybe 40 pixels or more if it's just the core (it really depends on the scale of the image). Use your judgement.
- Turn on the short exposure layer on the top
- Select the short exposure layer
- Click the Add Layer Mask icon at the bottom of the Layer Pallette
That should feather just the area that you want into the long exposure image nicely.
I hope that reads properly Mark, I only just woke up