Extremely Faded Diazo Print Restoration

[Gimpy222]

I have some old engineering drawings that were created using the diazo process nearly 60 years ago. The paper is yellowed with age and there is spotting from mold and mildew over the years. Most of the lines are pretty much intact, but in may places the text is illegible.

Originally the print would have had a light blue to white background with dark blue lines. Something like this.



I don't need to recreate the original "look", I just need to enhance the the text to make it legible. Ideally grayscale for printing on white paper. Background noise isn't an issue. This is a research project. Being able to read the faded text when printed more important than overall aesthetics.

I've experimented with the Levels and Curves tools. So, so results.  Looking for ideas or a process to get the best results. Thanks Sandy

Here is a sample from one of the sheets.

   

[Ofnuts]

Playing with the channels, there is more contrast tin the green channel. So

1. Copy the green channel to a layer (just drag frm the Channels list to the canvas)
2. Apply a moderate filter. The "Mean curvature" has the nice property to remove the spots without impacting the text too much if used with care
3. Add some contrast with Curves