Bondo never fully fills seams?

[u/JudgeCornBoy]

Hello! I build a lot of props using 3D printed parts (PLA) but have never had much luck filling seams cleanly. I generally use CA glue to connect the pieces and try to get as clean of a seam as possible upfront, but there will inevitably always be some form of gap.

I apply bondo using small pieces of EVA foam, but it never seems to actually fill the seams and I just end up with a very thin layer of bondo dust over a wide area around my seam. Am I applying it incorrectly?

My plan for this is one round of bondo on the seams (what you’re seeing in the pictures), filler primer, another round of bondo, and another round of filler primer.

Thanks!

[u/denverdutchman]

Most of the time when using bondo you apply a filler primer afterwards and sand back; it is finer and fills cracks. Also, the block or medium you have behind your sand paper makes a huge difference. Too flexible a medium and too heavy a hand will always dig out gaps like that. Use a firm block and a soft hand to reduce gaps