How to fill corner

Should have been addressed before the pieces were finished. Normally a latex caulk would be applied, tooled allowed to dry. Caulk is used due to expansion and contraction of the pieces/joint.

Tom
 
Yes, a flexible acrylic caulk should be used to make seams like that disappear. After you've painted it you won't see a thing.

Putty's no good, because it is not flexible, and after a while the seam will be back due to the movement of the wood.

When you apply the caulk, hold your caulk gun straight so you push the caulk directly into the seam. The seam in your pic is very small, so you need just a little bit. Then rub it in with a finger or a small brush, and then remove the excess of caulk with a putty knife and/or a slightly moist rag.
 
Andy UK said:
Yes we call it Decorators Caulk over here in UK.
Can be painted over once dry.

Yep decorators caulk applied with a mastic gun and then a quick wipe over with a wet finger, and it will look perfect.
 
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