Mending (5e cantrip-spell) has these limits:
that the damage must be a break or tear
the damage cannot be any larger than 1 foot in any dimension
the spell cannot restore lost magical properties
One (entrepreneurial) player of ours suggested that ANY two unrelated surfaces can be seamlessly bound, providing both surfaces were of materials formerly broken or torn (ibid). This spell does not specify any previous relation / connection - nor is Mending a Divination spell (this magic could not know which surface belonged where previous to a casting). In fact, this spell would not even know what was 'torn' and what wasn't - implying ANYTHING could be seamlessly welded onto virtually anything else.
We pointed out we have friends in StackExchange and he would not see the last of us / this was not over / do not rest too easily &/or comfortably.
He went on: 'even with any StackExchange requiring specific torn-relation necessity, one merely need affix unrelated parts first, with any glue - once said glue is broken one is ready for a Mending weld-meld'. Example: take two polished steel parts... add Elmer's® Wood Glue... let dry / snap connection... NOW you can set-weld these two parts perfectly with a magical Mend. (Unrelated: our DM went mildly non-linear at this proposition.)
Quick &/or Dirty Summation:
How related must two surfaces be in order to connect-affix-meld a square foot seamlessly with the Mending spell?