You should also be carefull about pressing the paper of the painting against the glass because this causes 'foxing.' Basically, the glass is an insulator, and with natural humidity and changing temperatures, condensation builds up in the inside of the glass. It just does. Pyhsics. Over time (shorter time if displayed on an outside wall), the moisture from the condensation develops ugly blackish gray mildew spots which embeds in the paper.
I suggest you attach the painting to the acid free backing, but install a mat with an opening cut larger than the painting that is also thick enough to keep the artwork from coming in direct contact with the glass. Double thickness matts are useful for this. The first of the two mats need not be visible from the front if the opening is cut larger. Use acid free hinges, available at any quality frame shop or made with acid free tape. This is museum quality framing.
Museum framing can be done yourself, and at only a bit more cost in materials than framing that can cause real damage to the work-under some conditions this damage can be permanent and rapid. How much is the painting worth? I usually figure if it is worth framing at all, then it is worth the added expense.
There's my two cents.
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