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Relatively new to 3D printing but have learnt a lot, I just need some help on the finishing process. I currently have some print-in-place PLA figures I need to sand down and some have holes I need to fill in (we made a bit of a mistake on layer thickness). I have a fair amount of hands-on experience with sanding and filling from my tenure making sculptures from foam/cardboard and the like but I'm unsure on the materials for plastic.

That being said, is regular poly-filler or wall filler okay? This is what I've used before while making sculptures, so I have the most experience with it. Or should I be looking for something more specialist or specific like using a two-part resin for filling and sanding that down? (I do have access to a well ventilated space I can do this in.) Should I simply go over everything with a soldering iron?

These figures will be primed and painted afterwards, so I would prefer if the filling material was relatively easy to paint over. Colour doesn't matter as long as it gets the job done though. :-)

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    $\begingroup$ The same PLA through a 3d printing pen $\endgroup$
    – towe
    Commented Aug 9, 2023 at 14:15
  • $\begingroup$ @towe I don't have a 3D pen, should I use the soldering iron instead? $\endgroup$
    – Nathan
    Commented Aug 9, 2023 at 14:46
  • $\begingroup$ Soldering iron works for adding material and welding it into the print, but is a pain to use. If you can get a 3D pen that's so much easier. $\endgroup$ Commented Aug 9, 2023 at 16:51

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CA Glue

for very small holes or print defects, I like to use a droplet of CA glue, which fills those small defects.

In combination with baking soda, it can fill even larger gaps and voids. Glass or plastic pearls and sand also can work as material for filling, as can sanding dust.

Filler-Primer Spray

While not useful for moles larger than about half a millimeter, filler primer sprays can cover over and hide dimples, scratches from sanding as well as layer lines.

Filler paste

Be it Plasto, Tamiya putty or wall filler, small to medium defects and dimples can be filled with these compounds. Plasto in particular has a rather aggressive solvent in it, that strips through paint, so keep that in mind. Among similar products, Tamiya has a range of filler pastes I have good experience with, very similar to Plasto, and is available in different styles for different plastics. In even larger packaging, 3M 05096 is a very decent acrylic filler and comes in industrial-sized 411 g/14.5 oz compared to the about 30 grams of hobby fillers.

PLA soldering

If you have a clean soldering iron or 3D pen, you can use spare filament pieces to melt and fill gaps or even join parts.

2 Component Resin

A favorite of mine for smoothing over organic surfaces is coating them in two-component resin and letting that dry. The result is usually super smooth, but this is not useful to fill in larger holes that need to be built up first.

"Greenstuff" et al

A better alternative for filling up medium to large holes in a print is using a material akin to Greenstuff, Milliput, or any other glue putty, such as Pattex Repair Express, Uhu Epoxy Repair All or many others. These two component resins start as two strips of gumlike consistency and after mixing turn into a hard polymer over the next 5 minutes to 12 hours, depending on the formulation. This stuff can make a structural replacement for even the largest holes, and be smoothed mechanically.

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    $\begingroup$ If you're going to be filling with CA glue, it works even better to pre-fill the gap with baking soda, then drip low-viscosity CA glue on it. The glue instantaneously hardens and produces a more substantial "fill". $\endgroup$ Commented Aug 10, 2023 at 17:28
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    $\begingroup$ In my model airplane building period we used glass bubbles to mix with the CA or Epoxy. $\endgroup$
    – 0scar
    Commented Aug 10, 2023 at 20:26
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I've had good success with UV-cure glue as a filler. It's reasonably hard when cured yet sands easily. It takes paint well. It has the advantages of unlimited working time and a very quick cure time.

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