For comparison reasons, see below the same print without spray and thus without the grey haze.

I used a cloth to clean the sprayed print and I was able to completely remove the grey haze particles without scratching the print. That is good news. Unfortunately, I noticed that, despite that the grey particles were gone, the print was noticeably less bright and less saturated than the second identical print I prepared for comparison reasons. See the image below.

The picture I show here to illustratet he difference, may not be a perfect shot due to the poor light (unequal distribution of light),  but I am sure you get the picture. Trust me, the sprayed print is noticeably less saturated and darker than before the spray was applied.

Interim conclusion

I consider the decrease of brightness and saturation - meaning less gamut in your prints once sprayed with the UV Protect spray - a big problem. Especially with the HP T120 / T520, with their already limited gamut. But for any printer, a reduction in gamut is a high price to pay. It also means you need to make dedicated ICC profiles from prints after the spray is applied, to compensate for the impact of the spray on the colors.

If, and only if, the spray turns out to highly extend the colorfastness of the HP T120 / T520 prints on microporous papers such as the HP Instant Dray Satin and Gloss papers, this spray can be considered (dispite its impact on the gamut). In that case, we dye ink users, will at least have an option to produce reasonable colorfast prints again now that HP has dropped the production of their swellable HP Premium Photo papers.

Further tests

I prepared some other test prints on plain paper, HP Advanced Photo paper and HP Instant Dry Satin Photo paper (with and without Lascaux UV Protect 3 spray) and mounted them to my window to see what the long term (a couple of months) effect of direct sunlight (although behind window glass) will be on the these test prints. I will report back in a couple of months on the results.