What’s the real difference? Pretty simple really, UV stable polyurethanes (or aliphatic to the scientists out there) maintain their clear colour and are not affected by UV radiation, whereas Non-UV polyurethanes (aromatic to the scientists) become a darker brown colour when exposed to UV light. So if you want a resin bound surface to stay the same colour, you need to go UV. Also, it is paramount to use UV if you are using a light or coloured aggregate blend, otherwise the aggregate you choose will be nothing like the finished article. Some argue that Non-UV is fine if the aggregate blend to be used is yellow/brown, however, unless the surface is completely evenly exposed to UV, there will still be considerable colour variation due to variances in shading (which is highly likely; parked cars, gates, trees, plant pots etc.). Some sources indicate that Non-UV stable systems can become brittle over time due to UV exposure, which may well be true, however the industry started out as completely Non-UV and proven to be strong and durable. In fact, Non-UV resin is often much stronger than UV stable resin. Moral of the story, use materials which have been proven to have a good track history. One thing to be aware of is people selling 'UV Stabilised' resin. This is likely to be an aromatic polyurethane (Non-UV), stabilised with UV inhibitors. Now, although the inhibitors will slow the discolouration process, it will not stop it. Again, if you don't want the surface to change colour, make sure you use aliphatic - UV Stable polyurethane!