I think JBM nailed it...I don`t think many of these tire dressings offer any kind of protection and if they do it`s very short lived. A cover is your best bet, otherwise I would go with the 303 Aerospace protectant. Tires are also meant to be driven on and without it, the tires aren`t able to archive the blooming process.
RaskyR1 hit on some thing about tire "blooming". This is that "brown stuff" that every detailer sees when washing tires with an APC or degreaser to clean them. It happens because the ozone in the air reacts with chemicals compounds/agents in the tire specifically designed to prevent cracking and aging of the rubber, as ozone and heat will deteriorate rubber compounds over the life of the tire. It is not caused by tire dressings or brake dust! That said, dirt and brake dust will adhere (cling) to greasy-type dressings (AKA, silicone) and they will appear brown or dirty.
Anti-ozone compounds in tires take two things to work there way to the tire surface; tire air pressure and heat. You can find that a new unmounted tire that has sat in a garage or store for a while will NOT be brown because there is no tire pressure INSIDE the tire to force out this anti-ozone compound out nor has it been driven, and hence, subjected to heat generated from normal vehicle tire motion (braking, cornering, towing or hauling weight overloading, or high-speed travel).
The OP is concerned about UV- induced damage to the exterior rear-mounted spare tire, but ozone and heat are (much) more detrimental to a tire than UV. Will a tire dressing/shine help "protect" a tire from aging and cracking??? That`s a subject of debate here, but tire manufacturers have already "engineered" this into today`s modern tires. Making them "look" new is really what tire dressings are all about and I am of the opinion that "neutral or matte finish" is more desirable for a tire`s appearance.
I use Aerospace 303 myself on tires because it does look matte and it`s what I have available to me OTC and it`s relatively inexpensive, BUT it will wash off in the rain AND it will streak onto paint. (I know from using it on truck bed tonneau covers, it WILL streak onto the sides after a rain, even if you buff it off). Just food for thought if the OP wishes to forgo using a tire cover and uses 303 and can live with streaking on the rear tailgate or bumper after a rain. That may be true for any water-based dressing.
