I found it difficult to get the Print Shield spray here so I tried two readily available and cheaper sprays that are marketed as for photographs: National Art Materials Crystal Clear and National Art Materials Matt Spray. Both are made in Australia. I recognise the benefits of non-aerosol coatings, such as Breathing Colour Timeless, but for the moment an aerosol spray has benefits in convenience.
I printed off a 255 step black to white image on my P600 onto Epson Cold Press Natural, which is my main b/w paper. I then covered strips of the print and applied both sprays. The results are quite staggering. The Matt Spray kills Dmax to the extent that it has very limited use. The Crystal Clear, on the other hand, actually increased Dmax. While it is marketed as a gloss spray, there was no visible increase in gloss at all.
In the image below, the strips are: no spray; (m) Matt Spray, (g) gloss (Crystal Clear), no spray, (m) Matt Spray, (g) Crystal clear, no spray.
There was no visible colour cast (the bottom right appears slightly warmer, but I think this part of the paper was curled up shielding the cooler sky light).
I'll use the Crystal Clear spray in the future. Any info on its longevity would be appreciated.
Bob