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3D Printing : Smoothing 3d Prints with Bondo

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: Smoothing 3d Prints with Bondo
« on: Oct 12, 2018, 08:46 PM »
Smoothing 3d Prints with Bondo - or - Getting Rid of VPLs
Lead Author: Keely
Edited by: MMCC Education Team

3D printing is one of the best things that's happened to costuming in years! It makes it so easy to make complicated gauntlets, helmets, weapons and more.

But with ease, comes compromise - most 3D printed items need a good deal of sanding and prepping to cover their visible print lines (VPLs) and imperfections. This guide will show you how to masterfully disguise these blemishes with the use of Bondo spot putty and filler primer.

General Safety

While following this guide, you will be working with some tools that are sharp, and there is the potential to cut yourself.  Please keep a first aid kit on hand.

You will also be working with chemicals  (Bondo Spot Putty, and Filler Primer) that offgas toxic fumes. Please work in a well ventilated area, and consider wearing a respirator.

Please wear a dust/particle mask for sanding.

Gather Your Supplies

Required Supplies
  • Sandpaper (220 grit is the most common)
  • Bondo Spot Putty
  • Filler Primer
  • Paint

Suggested Supplies
  • Drop cloth
  • Exacto Knife
  • Mouse Sander
  • Miscellaneous Sandpaper grits (40/60/80, 120/180, 220, 500, 2000)
  • Sanding block
  • Files
  • Hook & Pick set
  • Baby Wipes
  • Gloves

Prep for the Bondo

Gather your 3D printed items. We'll be using a set of macrobinoculars as our example. Use your exacto knife to cut off excess plastic, and your hook and pick set to remove any supports.

You can see how much support material was removed from our macrobinoculars.

Use the file set to sand down any small areas and to get off any rough bits left from the supports

You can see the rough sanded parts below.

Now use your lowest grit sandpaper (40, 60 or 80 works well here) on your mouse sander to sand down any imperfect portions of the print and any sections that have very prominent print lines.

You can see easily some of the extra plastic bits that should be sanded down with the lower grit at this stage in the below photo.

Here are the pieces after the low-grit sanding.

Pick up a higher grit sandpaper for your mouse sander. 180 or 220 would be good here. Sand over all of the pieces. You'll be able to smooth the print lines a tad like this, but they'll still be present. Most of what you're doing at this point is making sure the edges are all smooth.

You can see how much smoother the pieces are now,

Now it's time to hand sand! 220 or 500 grit would be good here. Smooth around the edges a bit more and get into the crevices that the mouse sander couldn't get into.

If you have a dremel with a sanding attachment you can use that as well. If you have higher grit sandpaper (like 2000), you can do that afterwards too.

It's much smoother now! You can still see the print lines, but the rest is pretty smooth.

--NOTE-- If the item you're prepping has parts that need to be glued together prior to assembling (ie: you have a helmet that was printed in parts), you should glue your pieces together with CA glue (super glue) at this point. Sand the seam down so that it's even before applying bondo. If you have any holes to drill you should do that now as well.

The below photo is of a different project (Pit Droid, not macrobinoculars) to show you an example of gluing, prepping, and painting pieces

Now you're ready for bondo!

Bondo Spot Putty

Bondo Glazing and Spot Putty can be found at an automotive store, or often times at Walmart in the automotive zone. Unlike traditional bondo it is NOT in two parts and does not need to be mixed prior to using.

Wipe the bondo onto your pieces, coating the entirety and paying special attention to any imperfections on the print and any glued seams.

You can use baby wipes, paper towels or even just your finger to smooth the bondo on.

Set the bondoed pieces aside to dry for about 15 minutes.

Sand down the bondo. You can do this a few different ways - if you have several layers of bondo you may want to use the mouse sander with 180 or 220 grit. You can also hand sand.

This step will make QUITE a bit of dust! To avoid this you can sand outside, or you can "wet sand" in the sink or a bowl. If you wet sand, you'll need to be sure the piece is completely dry before priming.

Some of the print lines will likely become visible again after sanding. Many of these will be covered with the filler primer, but if they're really bad then you can put on a second layer of bondo in some spots.

Priming and Painting

Like Bondo Glazing and Spot Putty, Filler Primer can be found at an automotive store, or often times at Walmart in the automotive zone.

Spray the pieces in a well ventilated area and let dry a minimum of two hours, preferably overnight

You can see some print lines in a few places even after the filler primer has been applied. Take some more higher grit sandpaper (I used 220, but many use 2000 at this point) and hand sand the primer softly.

Sanding down the primer will often finish the camouflage of the print lines. If it does not you can do a second coat of primer at this time. If you have glued seams that are still showing or VERY visible print lines still, it may be a good time to add some more bondo on top of the primer in your problem places before putting on a second primer coat.

Once your print lines are covered it's time to paint! Use your chosen paint (if spray paint, be sure it's the same brand as your primer) and get to work!

Assemble your piece and add any electronics or fancy schmansy bits!


Every project will be a little different depending on how many details they have, the type of printer and filament used, and even the settings the  pieces were printed at, but the above steps should be a good starting point for any 3D print. With enough patience and elbow grease you too can erase those pesky VPLs!

« Last Edit: Jul 23, 2019, 09:03 AM by Raestin Ke'Varek » Logged

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