Old writing on back of painting - Does anyone have the skills to show the text?

I'd love to see the outputt using the techniques that D Fosse describes for filtering at the image capture stage, which are far more likely to yeild a usable result.

I had a quick look to see if there was anything in your attached screenshot  at all, and there is but not enough to read if the text is not already known.

I used an FFT filter on the red channel, then the camera raw filters texture (negative) and dehaze (positive) filters, before blending them with the original colour.

Dave

In your case I feel the most important thing is to kill the texture first of all. I'd try lighting it from all four sides, and experiment to find the best angles to the surface. Perhaps even try direct frontal light close to the lens. That's not something you would normally do, but might work here if it's not too reflective.

Anyway, I've done a bit of this (I work at an art museum). That's when I pull out my IR/UV-modified camera and bandpass filters. Sometimes you get closer, but it never just pops out. That's just on TV. UV is usually most effective.

Here's an example. Visible light, infrared and ultraviolet (reflected):