If you epoxy the bolts in, they will hold but you will never get them out.

so do this instead: epoxy the bolts in, but from the bottom. when they are set firmly, put the board on, with the nuts on top. If you need to remove the board, just take the nuts off.

You will want to lubricate the threads the nuts go over, or weather is likely to make them jam sometime later. Use a heavy grease or antiseize, or Teflon tape, either works well for this.