Digital BW, The Print

>...should I use Lyson Print Guard on papers like hahnemuhle William 
>Turner or Luminos Museum Parchment or Legion Somerset 
>enhanced?

Print Guard appears to be the same as PremierArt Print Shield, which has
been tested by Wilhelm.  See 
http://www.wilhelm-research.com/pdf/WIR_Ep9600_2003_11_01.pdf
Some of the papers were sprayed with PremeirArt, and it appeared to help the
lightfastness and not affect the paper life estimate to the extent of the
testing (to >200 years).

>What is the long term effect of Print guard. Does it eventually 
>crack or discolor.

These companies are not disclosing what the products are.  Solvent based
acrylics have the best reputations for not yellowing, and I assume that is
what these are.  My past research led me to Rohm & Haas's B72 formula as the
most trusted by conservators.  Grumbacher Tuffilm is apparently the same
thing and readily available at art stores.  

See the statements of Ross Merrill, Chief of Conservation at the National
Gallery of Art, Washington D.C. (USA), below.  In a 1997 speech regarding
pastel conservation he said as follows regarding fixatives (which these
are):

"Today there are a number of alternatives. Krylon Acrylic Varnish B72 is
used by conservators.  [Krylon no longer makes this, Lascaux Fixativ is the
only true Rohm & Haas B72 that I know of.] This product will remain
unchanged for 400 years, but does have a tendency to saturate the surface.
Grumbacher B77 varnish or "tough film" is the same as B72, but will not
saturate..." See <http://www.bmi.net/knapp/iapsmerrill.html> 

I prefer the Lyson or PremierArt for glossy prints because it is easier to
get an even spray.  For matte prints the Grumbacher or Lascaux has worked
fine.  Grumbacher is probably the cheapest and most readily available of the
products.  I think it has been used by pastel painters for many years.

Paul
www.PaulRoark.com