Our main mirror needed a cleaning before installation. This is what one does with water. For this purpose we lined the transport box, dissolved dishwashing detergent without additives and filled the pool with water. When filling up, one can cover the mirror with a powerful jet. After two hours the water was drained and the mirror was rinsed with distilled water. We then performed the whole procedure again. A faint residual gray haze can only be removed with a wet cotton ball without pressure. However, this should not be done, because such a 
process damages the unprotected aluminum surface and cancels the reflection gain. Apparently, the mirror does not currently need a new coating and can be installed.
well done!