Most 451 coupe owners will notice that tiny cracks appear in the plastic.
The cracks can't be felt as they are internal and although it's very rare, they can eventually allow water in.
This isn't limited to the 451 clear roof. It is also found on 450 rear side windows, Roadster coupe rear side windows,
Forfour (454) clear plastic roofs and will eventually also cause problems on the clear roofs of 453 models too.
The issue is that polycarbonate is a UV filter. It does this by absorbing UV radiation from the sun light.
This is good for the interior of the car as it's less susceptible to sun bleaching because of the UV light.
The sunnier/hotter your country, the quicker the roof will start to craze.
However, it's not good for the roof over the long term. The UV causes microfissures (small cracks)
which aren't necessarily a structural problem but it can make the roof look like crap.
Polycarbonate that is intended to be used outside will normally have a UV protective coating.
The cheapest option is a UV protective lacquer that is sprayed onto the outside of the plastic.
The most expensive option is a separate layer of UV protection is laminated to the polycarbonate.
Needless to say, the cheaper option is what smart use and the lacquer doesn't last for more than a couple of years.
There are no fixes for your existing plastic. There is no way to repair it. So you have 2 options.
Live with the cracks .
Fit a new roof.
Yes, if you just leave it like you did with your previous roof.
However, now you know, you can prevent it happening with a little work.
Again, you have 2 main options.
A spray on lacquer or a plastic laminate.
Spray on lacquer
I bought this 303 aerospace protectant from eBay. It's not the cheapest stuff in the world.
It's the gear that they use on polycarbonate plane windows so it's good stuff.
All you have to do is clean your roof, spray this on and wipe it dry immediately.
Reapply it every 2 or 3 months and your roof will be protected and should never craze.
Some companies make a UV film for windows or UV paint protection film for cars.
If you can find this stuff, it'll work but actually getting it to adhere to polycarbonate can be tricky.
If you have seen tinting film on fortwo or Roadster rear plastic windows, you'll see that it doesn't like to stick.
Thanks to Scott A for the screen shot of this video to show the cracks.