My wooden worm box is inside so the original olive drab army paint is on the outside of the box (decals, stencils and all). The inside of the box was painted with fiberglass resin which created an extremely tough surface inside. It will stand up to repeated turnings with a pitch fork or shoveling out of the vermicompost. Fiber glass will cost a little more than paint or polyurethane but will last and hold up to rough treatment. Wurmz iz e-z!

A wooden bin will rot from the inside out very quickly unless the wood is protected in some way from the inside.

The paint is for the inside and especially the edges or end grain. Give it 3 coats. If this bin is on the ground you should consider a hardware cloth bottom to repel gophers,ect.. A wooden bottom will rot very quickly and is not needed. The great thing about using scrap wood is it is FREE! Replace in a few years if necessary. I have a 3'x5'x18" bin I started last summer and just harvested 50 gallons of castings. Have fun!

50 gallons!

You are rich!

I dream of having enough vermicastings so I can add an inch layer to a small section of the lawn.

So far I have only 5 gallons of vermicompost not castings so I have to be very frugal with it and only use it to start seeds.

I am so thrilled at all the information in the replies. Thanks so much everyone!
I just completed my 'sunshine yellow' paint job on the box inside and out. When it's dry, I'll probably slather more paint on. I plan to have it up on bricks at a tilt angle to drain liquid out of holes in the front of one side towards the bottom.
I'll definitely put something on the bottom to repel the gophers, though hunting them is currently my cat's favorite sport.

I LOVE the 'free' aspect of the scrap wood. I figure I'll find a million 'improvements' to add into the next design...can never leave well enough alone... Thanks again!