I check every square that comes into my shop. First thing.
  Never allow a bad square to linger at all.
Squares can sometimes be fixed. I have had to repair a few squares 
including couple of starretts in my life.

   You first have to have a master square you absolutely trust. This is 
not as easy to prove as you might think. There are many surfaces on 
every square and any one of them might be out a hair, either though 
misuse or casual manufacture.
So it takes a while testing and retesting to make sure every bit of your 
master square is square.
      If you go though this, "retire" the master for use, except as a 
master square.

     Mine in an 1896 Starrett that I was unable to find any fault at all 
in.
I already loved and trusted the square, but after much testing I deemed 
it the master. Every square in my shop is checked against it.  It leads 
a sheltered life now.

   Fixing a square is often not really as hard as figuring out what to 
fix. This is not as easy as you might think. Is it really the joint? Are 
the legs genuinely parallel? Is the face truly square to the stock?
  Are you measuring against a bad part and thinking another part is bad?

   When you are finally sure exactly what is wrong with your square? You 
can go about correcting it.
Wooden stock steel blade squares can often have the joint reset.
  Most of the time these get dropped and closed up. Look for the tell 
tale dented corner where it fell.
  Most of the time you can whack them back open with a soft driving 
punch, a vise and a hammer. Light blows and keep checking against your 
master.
   Hold both your master and your patient together, up against bright 
window light and squint, as humans do.
  Make sure they are both clean. Errant contaminants can make a fool of 
you quick.

      If the rivets are tight (it takes quite a few light blows to 
move), then just use thinned adhesive blown in with air after you reset 
it. (a soda straw or more stringent air) because you don't really need 
much.  Thinned pva or water thin super glue here.

    If the rivets are loose pein them tight again. This usually won't work.
Like Claudio says, truly loose rivets make for a candidate you simply 
let go. Immediately.
  I won't even hang a bad square on the wall.
If it has considerable decorative value sell it. Otherwise toss it right 
now.
  Out, get it out before you forget, and it messes with you.

   I have run into a couple of occasions the square was never right. 
Unparallel steel is not damage through use. Finding the fault and 
correcting it with a file is slow work. But its do-able.  Keep checking 
and rechecking as you slowly bring the square to true.
  Learning to use a file with precision is not automatic at all. But its 
a skill worth pursuing.

  Steel squares get whacked in any direction.  Peining the corner either 
open or closed results in unsightly damage. Occasionally its worth the 
trouble because its unusually sized and you just like that size for some 
reason. But you better like it.
   Steel squares in standard configurations can be had for too little 
money to waste your time over.
If there is very much wrong with an all steel square, toss it regardless.
               yours Scott




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    Scott Grandstaff
    Box 409 Happy Camp, Ca  96039
    scottg@s...
    http://www.snowcrest.net/kitty/sgrandstaff/
    http://www.snowcrest.net/kitty/hpages/index.html