For my first few attempts at spray silvering my 28-inch mirror I used the glass cleaning detergent supplied with the Angel Gilding spray silver kit, but I wasn't careful enough throughout the process to get the optical surface as clean as needed - which needs to be perfectly clean. I didn't understand about cleaning the side of the mirror either. Bob ("Oregon-raybender") explained how important that is, and that using chalk was the best substance to use for cleaning. So I used both the AG cleaner and chalk for my next, and first fully successful coating, but it took hours to clean a mirror that big. This past May I only used chalk for cleaning and it works wonderfully well - plus it's much quicker using only one cleaning material.
As Bob also pointed out, a silver coating is soft and fragile. Midas really slows down the tarnishing process but it doesn't completely stop it, so even if you manage to keep bugs and dust off your mirror, it will eventually tarnish. I use my previous coating for 22 months. at which point it measured as 95% reflective in Red, 88% in Green and 78% in Blue - which shows it had started to tarnish. Eyepiece views were still outstanding though. I stripped this coating in May and applied a new spray silver coating then.
Also, I recently modified my scope to add Ed Allen's Forced Ventilation System. That was an extensive but fun upgrade, and I hope it will help extend the life of the current silver coating - it will certainly help keep the coating cleaner so I'm optimistic.
Aluminum coatings are much more durable than silver of course, and spray silvering makes the most economic sense for larger mirrors. But saving money isn't the only reason to use a silver coating - it's a fun process that results in a precision optical coating that will out-perform all but the most expensive enhanced aluminum mirrors. Plus, if a silver coating gets too dirty to use, you don't wash it - you simply strip it and spray on a new one.