Whats best to use to fill gap on back of frame.

[upload]

i picked up some black frames, polycore (plastic stuff) but the some of the corner joins on the back have gaps, Whats the best thing to use to fil the gaps ,to make them look nicer .
thanks.
putty filler ????
Any links would be great.

[prospero]

Try some solvent-free grab adhesive. The sort that comes in pump cartridges. I use some 'No Frills' stuff from Screwfix and found that as well as being a glue it also makes a good filler. The nozzle makes it easy to apply without getting it all over the place. You can sand it when dry and it takes water-based paint. Doesn't shrink either.
Not saying it will improve the integrity of a plastic frame join, but it won't make it any worse. Work it into the gap and smooth with your finger. When it's dry, touch up with black acrylic paint or a black marker. Quick buff with a duster and bingo.