When mounting a poster to display, decoupaging it to wood gives it a sturdy base and makes hanging simple. The most important thing when decoupaging wood is preparing your wood and knowing if it has been oil stained. Using water-based découpage, like Elmer's or Mod Podge, on oil-stained wood can cause bubbling, condensation and peeling. You must use an oil-based découpage, such as shellac, in these cases or sand 1/4 inch off the area you wish to découpage.
Wash your wood surface with a mild dish soap and water. Use one drop of soap to a quart of water. Scrub the surface with a rag, then wipe with a clean, damp rag. Allow to dry.
Sand the surface with fine-grained sandpaper until smooth. Brush the wood dust off the surface with a 3- to 4-inch synthetic bristle brush.
Remove any tape or gum tack from the back of the poster.
Position the poster as desired on the wood surface and lightly trace around the poster with a pencil.
Brush a thin layer of découpage over half the poster area. Affix the top corners of the poster in place, then press down the rest of the top half of the poster using the palm of your hand to keep the poster smooth.
Brush more découpage over the area that will be covered by the bottom of your poster. Smooth down the bottom half of your poster. Allow to dry for three to four hours.
Apply a thin layer of découpage to the top of the poster, covering all of the paper and the area directly surrounding the poster to about 1 inch. If needed, continue the découpage down the sides of your piece of wood. Allow to dry.
Repeat the previous step, applying one last layer of découpage to the project, then let dry.
Water-based découpage can leave brush streaks, so use a brush with synthetic bristles and don't over brush the surface. One way to know if your découpage is water-based is to see if it mentions water cleanup on the container.
Oil-based découpage can be toxic and flammable, so read all manufacturers' warnings and use caution and proper safety equipment when using.