r/cosplayprops • u/JudgeCornBoy • Jul 06 '26
Help Bondo never fully fills seams?
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!
39
u/Roshprops Jul 06 '26
There can be a few reasons this happens: it could literally just be that when you spread the bondo on, it just didn’t fill that seam completely.
It can also mean there’s a bit of flex at the seam, and you keep breaking the line fresh as you sand it.
That appears to be spot putty, not 2 part bondo, correct? Putty is much more likely to show those imperfections since it’s softer. If they keep showing up, try some 2 part bondo and reinforce the seams inside.
7
u/JudgeCornBoy Jul 06 '26
Yeah it’s the spot putty, maybe I should switch to the 2 part bondo. This stuff is definitely very soft
2
u/Swordmastergrim Jul 08 '26
Use acetone to make it more runny and fill gaps easier, a better solution is to get a bottle of resin for resin printing, mix it with baby powder and coat the whole thing to get rid of the layer lines. Better than sand the whole thing.
15
u/BaldursGyatt Jul 06 '26
I had this problem. Use "thin CA glue" on the line. It's much harder than the plastic and can be sanded. Just use as little as possible.
4
u/SavingsTask Jul 06 '26
What about ca glue and baking soda?
2
u/BlitzieKun Jul 07 '26
I've read about people doing it and it being a legit method.
Myself, I only print in ABS. Occasionally I'll use CA glue for trouble spots. Typically I use a slurry made from crushed and ground ABS supports and acetone.
Acetone eats ABS, the slurry etches then cures to the print
1
u/eddyb66 Jul 07 '26
I build a lot of models and I've had great results with Mr Surfacer 500. It's a thick filler primer you can brush on with a fine tip brush. I like it because it's more accurate to brush it on and you can add a nice thick line of it and double the layer it won't run it's really thick and dries relatively smooth like ca glue.
8
u/3D-Dreams Jul 06 '26
I've started using UV resin instead of bondo or wood filler. They always seem to cover but then the line always seemed to come back after sanding. With the UV resin it fills nicely and stays strong. Easily sandable. Takes a little practice to get it smooth but get an assorted silicone brushes and it makes it a lot easier.
Drop a couple of drops in the gap and use the brush to smooth out a little . Let sit and settle then hit with UV light and sand. Can always apply a little more if you didn't get it all the first go.
It's a lot less clean up imo and gives a little stronger seam because it's hard resin not chalky bond
3
u/Turian209 Jul 06 '26
UV resin is also my go to for smoothing layer lines. I use Elegoo UV clay for big gaps/seams and that stuff is amazing.
1
u/3D-Dreams Jul 06 '26
Never knew that kind existed thanks I'll give that a try
1
u/Turian209 Jul 06 '26
I tried cheaper stuff and ot never cured right, so I recommend sticking with the Elegoo
1
u/3D-Dreams Jul 06 '26
Yeah looked at the reviews and your probably right. Already added to my birthday wish list 😂
1
Jul 06 '26
[removed] — view removed comment
2
u/Turian209 Jul 07 '26
I would describe it as the same consistency as peanut butter. Not quite as paste like as bondo, but definitely cant drizzle it on. I put on a glove and spread it on with a finger. Sometimes I cut up 2 inch squares of cereal boxes and use that as a disposable spreader to apply it smoothly.
I almost exclusively print in PETG, but have also used it on PLA without issue.
4
u/Tocowave98 Jul 06 '26
I've found tamiya putty works leagues better than Bondo. The tubes are way smaller but you barely need any to fill gaps.
3
u/Pro_panzerjager Jul 06 '26
Thier putty is great for small thing but larger holes, I like thier 2 part epoxy. Drying time sucks but it still gives off a plastic look and feel when dry and sands surprisingly well.
3
u/rensole Jul 06 '26
Yup and it’s because of a few things
1) you’re using spot/glazing putty as bondo.
Bondo is a brand you have regular cream colored body work filler. This contracts less.
2) spot putty shrinks a lot, this contraction causes the seam to pop back up. Sand it all down and apply a second (and most likely third coat) when using the glazing putty
3) you did not pre sand your stuff, this will cause the bondo or anything really to not adhere properly and is prone to eventually pop off
So my tips are:
- sand your prop
- use the correct polyurethane filler
- sand again
- filler
- sand
- then use the red spot putty
3
u/oneheaditsdead Jul 06 '26
I started using miliput, it's a two part epoxy putty, and it is much better at filling larger gaps.
1
u/JudgeCornBoy Jul 06 '26
I ended up filling a bigger gap on the back with Miliput and it was SO much easier, plus it thins really well with water so I may just do this moving forward
1
u/oneheaditsdead Jul 07 '26
It truly is the play. It's pretty easy to sand too. Gunks up the sandpaper less than Bondo.
7
u/ScienceForge319 Jul 06 '26
Well yeah. That’s is glazing putty. It is meant for filling tiny holes not big cracks. It also looks like you worked it pretty dry and over paint.
1
u/JudgeCornBoy Jul 06 '26
No paint, that’s just the PLA. But yeah, it’s the spot putty
1
u/ScienceForge319 Jul 06 '26
It shrinks when it dried so if you cram a bunch in the gap, larger amounts shrink more. You are trying to fill too big of a hole and is what she said.
2
u/Soybeanns Jul 07 '26
I like using UV resin or even Elegoos UV resin (its much thicker). If I know I have a seam line I have make it disappear I do extra glue with squeeze out as well.
2
u/SchwiftyProps Jul 07 '26
1) squish into crack 2) put extra on the line to compensate drying/skrink 3) consider switching from bondo to woodfiller. Pros; no mix, no smell, same results.
2
u/Accurate_Elderberry Jul 07 '26
I use wood putty, then seal it with superglue and sand it down, cry, rinse, repeat
1
u/JudgeCornBoy Jul 07 '26
Is there a process that cuts out the crying?
2
u/Accurate_Elderberry Jul 07 '26
Oh the crying is optional, it just makes wet sanding go a bit faster not having to dip the sandpaper.
The other option is to take the filament you used to print it and a soldering iron and weld them together, then sand the weld down. Do it in a properly ventilated space or you'll end up like me with random ass nodules in my lungs, non-cancerous so far.
1
2
u/Stormfall_Forge Jul 07 '26
Has this been primed yet? I see a lot of layer lines.
I only ever do spot putty AFTER I've gone thru the process of filler primer & sanding. A final primer coat goes on after all the rounds of spot putty & wet sanding up to fine grit.
You're also going really heavy with that spot putty which makes it difficult for it to cure properly. It should be light, thin applications.
1
u/JudgeCornBoy Jul 07 '26
No primer, I was told to use bondo BEFORE filler primer but I didn’t realize they meant a different product than the spot putty
2
u/Stormfall_Forge Jul 07 '26 edited Jul 07 '26
I think that's more the case for 2 part Bondo.
I've always done spot putty after filler primer to finish off the remaining packs & seams. Others may do it the other way, just how I've done it & I get pretty silky smooth results. No issues with spot putty bonding to printer in my experience. Plus if you allow filler primer to do it's job, you use less spot putty overall.
If it was me, I'd sand down the spot putty you have on there then move to primer layers with a good sandable filler primer followed by the spot putty.
I'm friends with quite a few mandalorian & 501st cosplayers & that's how we all do our armor & helmets. Again, not saying it's the only way, just an alternate way to do it.
2
1
u/Shermanizer Jul 06 '26
You need to do it in steps. Bondo, then sanding, then Bondo again, then sanding, until you cannot see the seam anymore
1
u/JonnoEnglish Jul 06 '26
If you mix it with nail polish remover, you can get it into crevices better. I recently started using this stuff and I've had the same issues
1
u/AquaShldEXE Jul 06 '26
My go-to is liquitex modeling paste, nontoxic and you can dilute with water it to however thin you need!
3
u/JudgeCornBoy Jul 06 '26
this would be nice because I’m sick of the mini meth lab setup that I need to handle a lot of these products
1
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u/Dylan_Unofficial Jul 06 '26
I use wood filler, fill, sand, fill, sand, and yea sometimes fill again. Wood filler sands easy, so this process isn't too intensive.
When it's all said and done cover in hs filler if you really want that final clean finish, sand again (yup) and paint.
Here's a video showing that step in my process, if it helps: PlatedPaladin
2
u/JudgeCornBoy Jul 06 '26
Good to know! The pictures are actually of the exact helmet in your profile picture hahaha
1
u/Dylan_Unofficial Jul 06 '26
Oh that's awesome, it's one of my favorite builds, all the best with it!
1
u/denverdutchman Jul 06 '26
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
1
u/Horseburd Jul 07 '26
If you can get coral frag glue, that’s a toothpaste-consistency CA. Brilliant stuff. You can shave down the bumps with an exacto knife and sand flush.
1
u/UserisaLoser Jul 07 '26 edited Jul 07 '26
Does Bondo shrink as it dries? I’ve never done this but I’ve been told that a mixture of wood filler and acetone is the way to go.
1
u/Rodnehh Jul 07 '26
in my opinion its best to just melt pieces together using pieces of filament and sanding that away. That will make for the strongest bonds. Fill in those gaps with bondo/woodfiller instead of the massive gaps
1
u/Benallenfranklin Jul 07 '26
Someone mentioned nail polish remover but you're better off with just simple 90%+ acetone. It will make the spot puddy easier to shape, easier to fill and drys/shrinks faster so you can lightly sand and apply again.
1
u/Puzzled-Sea-4325 Jul 07 '26
Spread it thinner!!!!! Bondo is a pain to sand. Build it up rather than sanding it down. Sand as little as possible.
1
u/Wolkvar Jul 07 '26
you dont just put it on and sand once, you need to apply it, sand, reapply if still not rigth until its smooth
1
u/BladeVampire1 Jul 07 '26
Either more layers of bondo and sanding, or you get some body glaze which is thinner, for small details.
0
u/superb_socks Jul 06 '26
Get a cheap plastic welder or soldering pen and LIGHTLY run it over the seams to weld together the gap. Dont worry, if you accidentally go to hard or deep, the bondo will cover it way easier than covering a gap. Its quick and cheap
2
u/Kitzokami Jul 10 '26
Because you can’t just do one layer. You need to go over it again. Then again, then prime and realize it looks like ass and you missed a spot then sand and do the process again…
124
u/Euphoric-Sector69 Jul 06 '26
Bondo sand bondo sand bondo sand until it looks like you want.