Sure it will...especially if your plutonium isn't perfectly clean...or isn't perfectly pure...or comes into contact with other substances in air...or is in a hot room. Machining even pure, perfectly clean plutonium in the open air *will* ignite the metal, for example. Plutonium, after all, is pyrophoric.
"The trick is to keep it away from water and air. Gladware, perhaps?" - calenel
Here's the DOE's bad experience from using plastic to seal plutonium from the air:
http://www.ieer.org/sdafiles/vol_3/3-2/pudanger.html
Much of the plutonium in the DOE Complex is stored as plutonium oxide, which normally consists of small particles. If not contained, such particles can be easily dispersed and inhaled. Plutonium oxide has the ability to adsorb (to stick: see Dr. Egghead) water and organic molecules on its surface. If the container (and thus the plutonium inside it) is heated, or if chemical reactions within the container raise the temperature, any absorbed water on the plutonium may be released, building up the pressure in the container.. Pressurization can also occur when the adsorbed materials are slowly released over time. In addition, the adsorbed molecules are subject to radiation from the plutonium, which can chemically break them up (the process of radiolysis; see Dr. Egghead). Radiolysis can also cause problems in packaging materials; any plastic in the packaging, for example, may disintegrate. Unfortunately, the DOE wrapped and sealed many containers in plastic bags in an effort to keep out moisture. A breach of the primary containment would therefore put plutonium in contact with the plastic. Radiolysis of some types of plastic bags releases hydrogen and gaseous hydrochloric acid, both of which react with the container material and the plutonium metal. These reactions increase the risk of fires; some of them also release heat within the container. Such reactions in turn increase the risk that the plutonium will not be contained.