Assembling Yoke Piece with Bondo- Getting Stress Fracture Cracks

[Vader's Wingman[TK]]

So for this 850 Armor yoke piece, I've been applying a thin layer of Bondo then sanded it down using 3M 220 paper.  I've already done this 9 times.

 

But I noticed that there is always a stress fracture crack when the armor is handled and I can't seem to get rid of them!  I haven't painted yet but I'm wondering if it's the type of bondo that I am using or is something like this expected to happen?  The Bondo that I'm using is "Glazing & Spot Putty".  What other types of Bondo are builder's using?

 

Once painted, I'm sure that the cracks will reappear again and I'm starting to get annoyed with the entire process!  Anyone have any suggestions with this?

 

Thanks.

[gmrhodes13[Staff]]

Auto bondo/filler will crack if there is any flex, many of us used JB weld or Devcon to glue these pieces together.

 

The joy with JB weld is it won't crack and can be sanded so you can pretty much fill any gap. I did however finish with a coat of bondo/filler and it's held up fine.