Spots filled with bondo won’t stay filled

[valid fiber]

Hey all, I’ve been working on this mando helmet for a while now, I had to print it in pieces and solder them together and I’ve already sanded them down pretty smooth. The problem is I’ve been filling the soldering lines with bondo filler and sanding it down to try to get it completely smooth with no signs of the soldering and every time I sand the bondo it just turns to dust and falls right out of the groove it was supposed to be filling. I’ve tried multiple times on each groove hoping it would fill a bit more each time but it really seems overall to not be filling. Some spots are smoothing out a bit but overall it’s not even close to perfect. The next step was to coat it with a sandable filler primer and I’m hoping a few coats of that will take care of some of the big gaps as well but I’m wondering if anyone has any advice. Thanks!!

[round lance]

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[burnt tulip]

So run me through your process, are you putting bondo in the gaps and scraping off the excess and then sanding, or putting bondo in the gap and leaving it proud(higher) than the gap and then sanding?

[valid fiber]

I’ve tried both but originally I was spreading the bondo on with a spreader to fill the gap and get it as flush as possible, then sanding just to remove the excess and to level it out if the bondo was higher than the gap. But for some reason it always just ends up turning to dust and falling out of the gap when I sand

[burnt tulip]

Have you tried to use some filament and a solder iron to melt some PLA in the gap and then sand that? Please wear a respirator if you do this.

[valid fiber]

That could work, I might try that. What do you think about hitting it with the filler primer first to see if that will fill it enough? I’m not sure how thick the primer is as I haven’t used it before, do you think it can fill in the gaps?

[burnt tulip]

Filler primer only fills fine scratches etc, spray putty would be thicker, but I think if the gaps are about 2mm deep then you would need to fill them first, have you tried wood filler?

[valid fiber]

This is what I’ve been using

[burnt tulip]

Looks like the right stuff, I am sure Frank mention UPOL filler in a recent video

[valid fiber]

Wait what’s UPOL filler?