Do primary mirrors in large observatories undergo regular removal and re-coating of the aluminum? Why?
This might not be the type of telescopes your are thinking about, but as IACTs (Imaging Atmospheric Cherencov Telescopes) in the end also measure light in (or near) the optical range, their mirrors are of similar build. 

The important arrays (H.E.S.S., VERITAS and MAGIC) have, at least partially, mirrors made of glass or with a glass surfaces, coated on
the front surface with aluminium (Al) and an additional single protective layer (e.g. SiO$_2$ , Al$_2$O$_3$ ). But as they are exposed to the environment the whole time without any luxury such as protecting domes or similar, the coating degenerates pretty fast.

In the case of H.E.S.S., which is located in the Khomas Highland of Namibia, the mirrors have to endure all kinds of environmental factors, beginning with the sand from the dessert, extreme temperature changes between day and night to curious birds picking at their reflected images. So after five years the optical efficiency of the mirrors was so bad, that one could actually see ones hand through from the mirror. To prolong the lifespan of the instrument, the mirrors were refurbished with a slightly different aluminium coating. In the following image the time of the mirror recoating is marked with the blue shaded area.



Image source: https://www.mpi-hd.mpg.de/hd2012/pages/presentations/Stegmann.pdf

The degeneration for the mirrors of the other observatories mentioned above where of similar timescales and they also did some recoating campaigns. 

For further information see for example the following papers, which contain some overview about the different possible techniques: 


http://www.brera.inaf.it/~canestrari/pubblicazioni/pubbl44.pdf
https://arxiv.org/pdf/1406.0622


If you ever happen to be in Namibia, go visit the World's largest optical telescope:



Image source: https://www.lsw.uni-heidelberg.de/projects/hess/HESS/hessmultnu.phtml