The RAF Museum are doing a full check on the condition of their aircraft as part of the preparations for the 100th Anniversary of the RAF in 2018. I presume the intent is to do any necessary work so they all look their best for the Centenary.
The work is being done for conservation rather than anything else. These aircraft were built in the knowledge that they would have a limited lifespan, certainly not 70+ years. Usually as a production expedient things like protection against, for example, dissimilar metal corrosion were not built in at the time of manufacture.