Stefano is following the instructions for miters from when white glue was the primary wood glue in use. Pre - Titebond, when Elmer's was it. He probably learned it from the same old codgers I learned it from back in the 60's.
It is good practice - I still use it with TBI when gluing miters, but I omit the water.
Spread a thin bit on each half of the miter and let it sit for a few minutes. Then add a bit more to one side or the other and assemble then clamp, wiping off excess with damp rag.
Be sure the clamping does not distort the frame members. Be certain your wood is the proper moisture content. 6-8% for most of the US is where you need to be.
CSI - Wood:
How wide is the cross section of the frame members?
Species?
When you inspect the joints and find separation, is it always on the inside (heel) or the miter or the toe? This can indicate wood movement from drying.
Is there any visible dried glue in the open joints? Inside edges or outside edges, both or neither?
Are the miters themselves perfect? Test frequently and rigorously, by dry clamping 3 corners. The fourth corner should just touch - not open, not hitting hard. If you don't have that right, no amount or type of glue will correct it.
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