friends had it done on their white UPVC windows, victorian bay front terrace now charcoal/anthracite grey, looks great and was done in a day or 2 but i think before they had fully moved in, most of the time is spent on the masking.
i have no idea on the product used but i’m guessing there is some kind of key/etch primer or maybe just a scotchbrite all over to give good adhesion.
i would have it done myself if the windows were good and it suited the property, i had triple grazing fitted in my flat but had to have dark wood foil exterior to match the rest of the building and just had white inside.
re the guarantee, do they offer an insurance like flat roof companies? this way it’s not void if they close the business?