You might want to look into archival material in 
repairing the sketch. Most normal adhesives are 
acidic. Also some plastics will off-gas as they 
break down and damage the paper. These are common 
concerns for those involved in matting and 
framing art. Maybe get the advice of a 
matting/framing person. On the downside, the 
paper itself is probably non-archival. I'm 
probably not much help, I slept through most the 
matting/framing stuff in class :)