Blu-ray is more susceptible to damage, because the data layer is only covered by a thin protective layer. The protective layer is scratch resistant, but not scratch proof.
If a light-medium scratch occurs, it can usually be repaired because scratches often range between 5-20µm deep.
The protective layer is about 100µm thick, and many disc repair machines polish the surface away at a rate of 1-5µm/min.
This means that most Blu-ray can be repaired in a matter of minutes on a compatible machine (which is most of them). Plus, several repairs can be made before the protective layer is too thin.
When it can't be repaired:
* A really deep scratch that has damaged the data layer.
* If the disc is cracked.
* When the protective layer is already too thin from several previous repairs.
* If the repair machine is not compatible.
There many stores out there with disc repair machines capable of repairing Blu-ray - you just need to ask your local gaming, music or video store, or search for Disc Repair Listings.
If you looking to buy such a machine, you just need to search for a Disc Repair Machine - the manufacturer will tell you if it is Blu-ray compatible.
Although some people claim to have success with manual cleaning using gentle abrasives, if the damage is anything other than very light, you will not be able to get a consist level of repair and generally either make the damage no better, or possibly worse. There's a good reason why stores spend good money on professional machines - because they know manual methods and cheap machines don't do the job fully.