Firstly, make sure the surface is well cleaned - ideally with an All Purpose Cleaner or a wipedown product.
You've a few options in terms of treating it and protecting it.
- Traditional type products are water / gel based, and are a matter of applying it quite regularly, every 3rd or 4th wash - Take your pick
- CarPro PERL - is water based, but contains silicon oxdie, and thus is more durable than the traditional types - its ideal on Plastics, Engine, Rubber & Leather, as it can be diluted down to whatever purpose you are using it - tyres mix it 1:1, engine bay / exterior plastics & trim 1:2 or 3, etc.
Another in a similar vein, of being water based and durable, would be KKD MistX (cant dilute though)
- Nano coating like CarPro DLUX are ultra durable coatings - so apply once, and wont have to worry about it again for at least 12 months. Hence needing such a small amount, but pricey - the benefits of not having to repeat applications is the biggest advantage here