Does thining wood filler make it crack when drying?

Question

I'm wrapping my structural beam and posts with 1"x "white wood" cut at a 45 deg and butted together to make my corners. well the problem is my crappy table saw and not so good table saw skills, Its not real bad but there are some areas that needed wood filler and most of them where pretty tight so I diluted the "plastic wood" (I think) putty with acetone. Every time I wait for it to dry and come back to it cracked and falling out, like its shrinking maybe?

Any help getting these gaps filled would be much appreciated!

Answer 1

For trim work that will be painted, use paintable caulk instead of wood filler. It does not crack and it is very tolerant of shifts in the pieces, both due to structural movement and seasonal swelling and shrinkage.

Wood filler is better suited to holes and divots fully contained within a single piece of wood.

Answer 2

Of course. The extra water will evaporate, without the volume the wood filler can't stretch across and it will crack. Basically wood filler will always crack unless the gap is tiny or you can keep the room still with the same humidity and temperature. You will need something over the wood filler if you have a larger gap that is more elastic and handles temperature changes better.