Spot White separation

You should talk to the people who will do the large format printing work. Different printers and large format RIP applications may vary on how art files need to be set up for printing white ink as a spot color or “sandwiching” layers of white ink and layers of color for backlit displays and signage.

Generally, any objects intended to be printed white will need to have a custom named spot color assigned to it. For example, I’ll use a user-defined “Spot1” named color to apply to objects that I want Onyx Thrive to output in white ink. I usually have to set the fill to overprint and even do counter-intuitive things with object stacking order. Anyway, it’s best to talk to the people who will do the print work.

Just to add to the advice already given by ​@CORCTON34033231 , it's quite common in prepress to use a contrasting colour for Spot White rather than displaying it as white in Illustrator.

For example, you could create a Spot White swatch and make it ORANGE (in this case) on screen. That makes it much easier to identify the areas intended for white ink, particularly against a white artboard or transparent areas. The actual colour you see in Illustrator isn't necessarily the ink that gets printed. The printer's RIP maps the correctly named spot colour to the white ink channel, so check the naming convention with them.

For the fading sections, I'd use tints of the same spot colour rather than reducing the object's transparency. For example, a gradient from 100% Spot White down to 0%, keeping the object's opacity at 100%. It's worth checking with the printer how their RIP handles spot-colour gradients.

I'd also establish exactly what the client means by a frosting effect. A solid white ink isn't necessarily going to produce the appearance of etched or frosted glass. The opacity, screening and ink coverage will all influence the result.

One other consideration is whether they're printing on the front or reverse of the glass, as that can affect the artwork orientation and print sequence.

And I'd definitely recommend a physical proof, ideally with several different percentages of white ink on the actual glass. What looks like a smooth fade on screen might produce quite a different result once printed.