Filler and putty: how to fill the layers on a print

What filler does

In FDM printing small grooves remain between layers. Filler fills them to level the surface before paint. Wikipedia writes about putties in general. For plastics, the main ones used are thin body fillers, putties for plastic and spray filler primers.

Filler primer is a spray coating with a high content of pigment and solid particles. It fills fine grooves and at the same time is the base under paint, see the entry on primer.

How to apply it

• Sand first. Sand the surface with a coarser paper so the filler grips. See sanding.
• Thin coats. Apply the filler thinly with a spreader, let it dry and sand it. A thick layer cracks and shrinks.
• Repeat. A smooth surface needs several rounds, each with finer paper.
• Small flaws. Gaps and holes can also be filled with superglue and baking soda, which makes a hard mass for repairing small defects.

What to watch for

Filler adds thickness. It is not suitable for joints and precise parts, since pins and holes can narrow by a tenth of a millimetre or more. For PLA and PETG choose fillers suitable for plastics and always test on a hidden spot, as some solvents will dissolve the surface. Ventilate while you work and wear gloves.

Filler will not help with deep grooves or large steps left by supports. It is better to sand those down first. Filler is for fine levelling, not for covering a bad print. It is therefore worth turning the part at the design stage so the visible faces come out as smooth as possible.

When ordering

We do not fill parts, you get them as printed. If you plan to fill, choose higher strength on the thin parts and expect the surface to be a little thicker. If a smooth surface is key for you, write to us before ordering at jakub.novak@boostello.com.