How do you stain an indoor wood bench for outdoors?

If there is a finish on it you will have to remove that,use a exterior stain and seal with a marine grade exterior varnish

HI Danielle. You will need to use a stain for outdoors. Then I would also seal it with a sealer for outdoor wood.

Stand off any existing paint, or stain. A light sanding, otherwise, to help the stain adhere. Outdoor stains can be found at almost any hardware store, and there are several tones, colors, and finishes, available. Ask the clerk to help you get the best brush, for the job. Adding a clear topcoat wouldn'the hurt, but make sure that it's an exterior topcoat. Good luck!