How to remove dust from your mirror without scratching
I store the six inch finder for my big scope in the garage. I didn't notice that the dust cap had come off until after cutting and sanding some boards one afternoon
Since the mirror is siliconed to it's wooden mount, I couldn't just soak it, so this is the can of Pre-Cote Blue Spray I used to clean it.
I got it from Universal Photonics
You have to pay a hazardous shipping surcharge, so I got two cans. (just a customer - usual disclaimer)
The process of cleaning a mirror is similar to using USP collodion, but it should be a bit easier to locate the materials
Wrap the mount in papers then spray a thick coat and let it dry for about 20 minutes.
After reading the MSDS, you'll know to spray outdoors or in a well-ventilated area, away from sources of ignition. It will be dull when dry. Runs or sags in the finish don't matter - get a thick coat so you can remove it in one piece
This is the Fun part
Using a piece of tape, touch the coating near an edge, then gently lift it off the surface. It should come off in one piece. If it leaves any behind, lift it off with a fresh corner of tape
Last Little Bit
You will likely leave a blue ring around the bevel. Lift it off with a fresh piece of tape. Here you can see some water spots - this technique removes dust but not stains
The dried coating seems fairly harmless - it's actually hard to light