How to use mirrors outdoors?

Where are you planning on placing the mirror? If you are planning on putting it on the side of the house or on a shed or other outdoor building, use stainless steel screws to attach it. If you are hanging it where it could possibly swing,make sure it isn't going to crash into something because picking out broken glass in a yard is not much fun.

Glass is generally pretty impervious to the elements unless it gets shocked by sudden temperature swings, so avoid hitting the mirror with cold water from the hose on one of those hot days. Again, shards of broken glass can get buried in a yard fairly quickly and bare feet and broken glass don't mix well.

Plastics aren't all the same and those that are UV-treated or neutralized will last far longer than others. Plastics can shatter and leave shards that are just as nasty as broken glass, so I'd look for a wooden or metal frame. You can paint the frame with several light coats of your chosen paint and attach the mirror using silicone glue or, if this is a wooden frame, glazing points. The silicone will hold the mirror to the frame well and be weather resistant.

You can get glass cut-to-fit from local glass companies. Search for "glass replacement near me" via the Internet.

I would avoid letting water/dust/leaves getting behind the mirror, because the slivering is not meant to be exposed to anything that will damage it.

Keep it under an overhang/eave or put a piece of flashing on the wall above it, that will drip away from the backside.

I love mirrors used outside, hang on a wall or the side of the house in a patio area.

What about checking out some local thrift stores? I would recommend adding a sealer to the frame to protect it in the elements outside.

I have a resin mirror in my backyard and it's held up very well.

go to thrift store and buy old vanity or dresser that has mirror it'll be cheapest way to get big mirror if it has brackets to bolt onto dresser just cut them off or remove and redo wood that is left around mirror painting it will be cheapest use wire cable to hang it with heavy duty molly bolts. where are thinking you want to use it? Patio,garden?? covered patio ceiling to reflect light downward? https://www.youtube.com/watch?v=gwJMl5R6rdg more https://www.youtube.com/watch?v=IMM-krNeZFk more https://www.youtube.com/watch?v=oX39hFCAoU8 more https://www.youtube.com/watch?v=7LDyDh_doOw weather proofing it https://www.youtube.com/watch?v=iQ7_xJAI9hY mirror paint for glass https://www.youtube.com/watch?v=v693FK6SyFw spinners https://www.youtube.com/watch?v=yXFrOKabvbg decorating them you could use Liquid nails construction adhesive,E6000,Gorilla glue outdoor adhesive,Loctite quick grab all come in waterproof