I watched a video on how to restore old photos (but can't find it again). They did it by creating blank layers above the "original" and then making all adjustments there. After all the discussion, I now remember that the option to "sample all layers" was available to the eraser tool and that is what I clicked to be able to use it on a blank layer. It DID exist, because I have dozens of photos where I did exactly that, but now it has either disappeared or I can't remember where to find it. Here are the steps I followed. 1. Made a copy of the background layer. 2. Created adjustment layers for threshold, levels (never have been able to master curves for some reason), etc. 3. Then I selected the entire photo (but not the white border around it), inverted it so only the white border around the photo was selected. I clicked on save selection and named it "frame." 4. I created a blank layer above the copy of the background, named it frame and then went back to the menu and loaded my saved frame selection. 5. I selected the eraser tool (with sample all layers turned on) and began erasing all the yellowing, tape shadows, etc. from the frame. I did not want to apply a white fill because most of the photos have hand-written captions on them and I wanted to preserve that handwriting, since it was my mother's. When I erased, it left a white border (not a clear background with that checkerboard pattern in the background). 6. Then I deselected everything, created another blank layer to apply the healing brush (or whatever tool I was using) to get rid of the scratches, large blobs, etc. I continued creating blank layers and making whatever adjustments I wanted to "correct" the photo. The image below shows some of the many blank layers I used to correct a photo.
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