As long as we're talking about framing, keep in mind that a major source of damage is light. A piece can hang innocuously on a wall for years, and because it's being hit directly with harsh light, gradually lose its coloring. It may also become yellowed. Both of these issues are fatal. Incurable.
Since most of Nagel's serigraphs are large pieces, framing them under glass can make them heavy. This is why the really big sheets are framed under plexiglas.
As a word to the wise, if you intend on purchasing a piece and framing it, seriously consider replacing the glass with UV-proof glass or plexglas and hang it somewhere where the sun doesn't shine on it directly all day long.