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The UV cures the LOCA glue. Complete sequence is: Pre-swell the film to get blue shift and replay in green. I use alcohol with small amount of water. The amount of the water control the blue shift. Simply drop few drops on glass and lay the film on it, add another drops and add another glass. Wait few minutes and the sandwich is ready to be exposed by pulsed ruby. For CW is better to use TEA. You can skip this step if you use chrome bleach or you want to preserve original color. After exposition I develop it roughly 2min in SM6 (for CW work CW-C2 should be better) and bleached it in Brilland bleach. After slow and complete drying I drop small amount of LOCA to clean glass, lay on it the hologram and press it by another glass. Then expose it to UV to cure the LOCA glue - resulting with permanently attached hologram to glass. The LOCA glue have simillar index of ref. as glass, the interface between disappears.
See the result - H2 copy of this way processed pulsed ruby master, dandelion in my dirty fingers :-)

What exactly does UV on this film ? Do you use it on freshly developed / bleached film ? Is it acting like a « fixer » ?

The best way how to handle the holofilm250 I found use few drops of LOCA uv glue (cheap on ebay for cell phone repair) to sandwich it to glass and press the sandwich firmly, then irradiate it by the UV lamp (or direct sun). Then the hologram can be used as master from the other side, or simply paint the back glass to black. An acrylate based black spray also can be applied directly to the emulsion side.

The Liquitex Professional Spray Paint is water based and can perform perfectly:

https://www.amazon.com/Liquitex-Professional-Spray-Paint-Carbon/dp/B008LUIUXU?ref_=fsclp_pl_dp_3

Also by Montana colors:

https://www.sprayplanet.com/products/mtn-water-based-300-spray-paint-carbon-black

This one I did not test but for Liquitex it is OK.

I’m using holographic film from an antique Holofilm250PAR lot. These films does not like to keep the paint on it, probably due to flexibility, the paint tends to scale out ( damaging the hologram by the same way).

I simply like to idea that a chemical process exists to make it just perfect easily :-)

What difficulties have you had with paint? A solution may be to seal the hologram with epoxy or RTV (like Ron & Bernadette Olson use), and a piece of glass. Then paint the back. Use a piece of smoked or black glass and you may not need to paint.

We should add a section « Blackening » in the wiki part of the website to write this down :-) I’ve been struggling with laminoir or black paint for like 2 years with pathetically discussable results xD

I think that the fizzing was just part of the chemical process taking place.
The blackening agent did work quite well.

Okay, not sure if fizzing is a good or bad thing but I take this is not a desirable effect xD

I have tried the blacking agent you have described over thirty years ago, but I never tried it with IPA.
It did work and as far as I can remember, the film fizzes when placed in the solution.

Steven.