I glittered a poster board using Mod Podge. Now that it is dried I notice unevenness on it. Is there a way to smooth out dried glitter on a large poster board?
By glittering the board, you basically glued rough particles onto the board. If you try sanding it smooth, you risk removing the particles again.
To smooth the surface without removing material, you need to add more material. In this case you need to add several thin coats of clear varnish. Acryllic varnish is easy to handle and should bind very well with ModPodge. I propose using a very soft and wide brush for this. First cover the board (or big areas if the entire board is too big) with a fresh layer of varnish, then sweep the brush in long sweeps from one edge to the other over the still wet varnish. This brushes the varnish from the little peaks of the glitter into the valleys, filling them up with material and evening out the surface.
You need to apply several (probably 4 - 8) thin layers of varnish this way. Make sure each layer is completely dried before applying the next. If you slob one thick layer onto the board, it will look messy and uneven in a different way (read more here). If you plan to paint or glue something else on the board, do that before applying the varnish to seal all the layers in evenly.
You won't get a perfectly even mirror finish like that, but it will smooth out the worst bumps. If you want a mirror finish, you need to slightly sand the dried varnish between applying each new layer and then polish the final layer.