Cquartz can absolutely be used on trim. As can Opticoat 2.0, Opticoat Pro, Gtechniq C1 ect.
Using a "trim specific" product would probably very well makes things a little easier, but i think i know what your problem was...
Im willing to bet your wife didnt go through the step...well, stepS to get the trim clean enough for the coating to adhere properly to the plastic..?
IME, cleaning trim, even on newer cars, can be a PITA.
Try this: Prior to an exterior wash, Id use a fairly stiff, but soft bush (firm toothbrush works well) and some Zep Fast 505 degreaser (1:1 to be safe), or APC diluted 4:1 or so, simultaneously scrub/spray your trim. Go over it a few times, using fresh cleaner to flush out the dirty cleaner (keep the drips on your paint rinsed as you go, dont let it dry). Once you do that, wash your car like normal and dry it. After everything is dry come back to your trim and use IPA diluted in distilled water (5:1ish) and a clean light colored MF towel to clean your plastic trim, youll probably get black transfer onto the MF towel as you rub the trim with IPA, youll wanna clean your trim until the MF towel comes off clean (or mostly clean, some older trim will continue to blacken towels if they are corroded). Youll notice after that, your trim my look a little lighter in color, because youve stripped the plastic of any prior dressings or products that were on it and any oils from fingerprints, road grime ect. Plastic, being porous, tends to hold onto these products even after they "wear off". Alcohol can also lighten plastics (use diluted not straight)- no worries, your coating should get the plastic to the way its supposed to look and keep it that way.
If you dont get your surface really REALLY clean, a specialty product like a coating will have a hard time adhering to it- thats my guess as to what happened to yours.
That being said, using a products specifically developed for platic restoration/protection may yield better results anyway since they are formulated to darken the plastic more than most paint coatings are.
Hope this helps, i havent used a coating on plastic, but ive cleaned plenty of it, shit takes forever to get all the way clean, and usually once you get it there, the trim looks more faded than when you started and its requires some kind of dressing to get looking correctly, repetitive use of the dressing will help restore its color, a coating/permanent product like Gtechniq C4 will make it look new for 2 years. I have a bottle of C4 dressing sitting in my Detail room ready to go on the 5.0. Ill be using the above mentioned method to clean all my plastic parts prior to coating.
OH- also, another product you may want to consider is 1z's "Deep Plastic Cleaner" Ive only used it in interiors and it works well, it may also work well on exterior trim- id use it in place of IPA- in fact, i may try this this weekend.
