The Basics of Post-Processing

FDM Printing will often result in artefacts such as scarring and layer lines. Post processing is recommended to achieve a finished final model.

0.0 Post Processing Methods

Post-processing refers to a series of procedures which can be applied to prints after they have been printed to achieve better results. As FDM printing is the result of melted layers of thermoplastic (PLA) printed upon each other, layer lines are often visible on the exterior shells of models.

Scarring from supports, and an uneven finish on the raft surface are other elements which will need to be post-processed to achieve a high quality model.

1.0 No-Sanding Method

The no-sanding method is the quickest and easiest method to achieve a decent level of finish to PLA prints. Aesthetically this process will not cover up the striations on your 3D-prints, however it is a great method for quickly colouring and finishing your parts to convey desirable effects.

1.1 Summary

Category

Rating

Speed

★ ★ ★ ★ ★

Finish Quality

★ ★ ★ ☆ ☆

1.2 Materials

  • High-cover aerosol paint + primer (such as Rustoleum 2X)

  • Clear Satin Fast-Drying Polyurethane (such as Cabbots or MinWax)

  • Isopropyl wipes or similar to clean part

1.3 Method

  • First ensure that the part is clean and dry. Wipe the part down with Isopropyl wipes or a similar product.

  • Apply a medium coat of high-cover paint to the part

  • Immediately apply a light coat of the polyurethane

img

  • Wait 10 minutes in-between coats

  • Repeat steps 2-4 a total of 5 times

img

  • Leave the part to dry overnight

  • If the part is discoloured, it may benefit from a final coat of high-cover paint.

img

  • Once you are happy with the finish of the part, apply a coat of Aerosol Clear Coat to finish your print.

2.0 Sanding Method

Although a time-consuming procedure, the use of a sanding method will result in the best possible finish on FDM printed parts. Relevant Personal Protective Equipment (PPE) such as a mask and goggles need to be used if utilising this finishing method.

In order to speed up this process, you can use a Dremel tool (or similar) in order to carry out the sanding. This is particularly useful for sanding parts with complex or organic geometries as there are various Dremel bits you can use.

Important Note: If you are using a Dremel or any rotary tool to carry out the sanding, it is important to do so extremely gently and take your time with it. The use of rotary tools will definitely speed up your sanding process, however it can affect the accuracy/tolerance of your print if sanded too aggressively. Apply consistent and light pressure with rotary tools in order to avoid removing too much material and ruining your parts.

Rotary sanding tools and sanding paper is available via the MSD's Machine Workshop:

2.1 Summary

Category

Rating

Speed

★ ☆ ☆ ☆ ☆

Finish Quality

★ ★ ★ ★ ★

2.2 Materials

  • Wet-Dry sandpapers of varying grits (80, 120, 240, 1000, 1500)

  • Pliers and Flush Cutters

  • Putty/Hobby Knives of Varying shapes

  • Small container for water

  • Respiratory Mask

  • Safety Goggles

  • Gloves

  • Isopropyl Wipes or similar to clean part

  • Automotive Filler

  • Aerosol Primer

  • High-Coverage Aerosol Paint

  • Aerosol Clear Coat

2.3 Method

  • Remove all support materials, rafts and other artifacts from the part

  • Use the flush cutter, pliers and knives to remove any unwanted pieces of the printed part

  • Dry sand the part, working up from 80 to 240 grits

  • Spray the part with light coat of automotive filler to fill in any gaps

  • Dry sand the part, working up from 80 to 240 grits

  • Once the part is smooth, Wet Sand the part, working up from 240 to 1500 grits

  • Once the part is completely smooth, clean the part with isopropyl or alcohol wipes

  • Repeat steps 3-6 as necessary

  • Apply a coat of primer to the part, follow all manufacturer safety directions and instructions

  • Apply several thin coats of high-coverage aerosol paint to the part, waiting thirty minutes between each coat

  • Once satisfied with the colour and coverage of the part, apply a thin coat of clear coat and wait to dry

3.0 Spray Putty/Filler Method

This method is on par with the No-Sand Method in terms of quality of finish but is a slightly slower process, however it is faster than hand-sanding or using a Dremel to post process your prints. Relevant PPE such as a mask and gloves should be used when utilising this method.

3.1 Summary

Category

Rating

Speed

★ ★ ★ ★ ☆

Finish Quality

★ ★ ★ ★ ☆

3.2 Materials

  • High-cover aerosol paint + primer (such as Rustoleum 2X)

  • Acrylic or enamel fast-drying spray putty (such as Superfine High Build Filler)

  • Isopropyl wipes or similar to clean parts

3.3 Method

  • Ensure that your printed parts are clean and dry, wipe the parts down with Isopropyl wipes or a similar product and let dry.

  • Apply a thin coat of high-cover spray-putty to the parts. Note that the spray-putty may cause blotching to your surface if applied too thick or unevenly across your surface.

  • Wait 15-30 minutes (or as instructed on bottle) between coats.

  • Re-coat your parts a total of 3-5 times until you have achieved even coverage across your parts.

  • Once you have applied all the necessary coats (according to how smooth you would like the finish of your surface) leave your parts to dry for 2-4 hours.

  • For additional smoothing/better finish of your part, dry sand your parts using 320-500 grit sandpaper.

  • Wipe parts clean using isopropyl wipes once sanding is finished.

  • Apply a medium coat of high-cover spray paint to your part.

  • Wait 10 minutes between applying your paint coats.

  • Repeat the painting process 3-5 times as necessary.

  • Once you are happy with the finish you have achieved, leave your parts to dry according to manufacturers instructions.

4.0 Epoxy Finishing Resin

This is one of the easiest methods to finish your 3d-prints and fill the striations in your print to get a smooth finish. The process is quite fast and will usually leave you with a gloss finish as epoxy resins are used for finishing surfaces such as timber floors. However, this can be easily worked around by spray painting your parts post smoothing in the same manner you would in previous methods.

One of the drawbacks of this method is the price of the product, it is not cheap but can be readily found in hardware and hobby stores as well as online. Aside from that the method is quite fast and produces a decent finish level.

This method is useful for post processing prints with complex geometries where it would be difficult to get to all the parts with a spray application.

Pro Tip: Sand your parts (working from 240-500 grit sandpaper) very lightly prior to applying the epoxy. Sanding it first will make the process of smoothing the surface of your parts easier and yield a higher finish quality.

4.1 Summary

Category

Rating

Speed

★ ★ ★ ★ ★

Finish Quality

★ ★ ★ ☆ ☆

Figure 4.0 - PLA smoothing using Epoxy Finishing Resin

4.2 Materials

  • General finishing Epoxy Resin (such as Z-POXY PT-40 Finishing Resin)

  • High-cover aerosol paint + primer

  • Isopropyl wipes or similar to clean part

4.3 Method

  • Ensure that your parts are clean and dry, wipe part down with isopropyl wipes or similar.

  • Ensure that your part is not sitting directly on top of the table or surface you are using to work. This will ensure your part doesn't stick to your work surface but will also allow you to access all sides and corners of your part evenly. So sit your part off the work surface by using a sacrificial block or similar.

  • Pour your resin into a little plastic container, then pour the hardener onto the resin in order to complete your epoxy. Mix thoroughly and begin using mixture straight away.

  • Using a brush (the size of the brush you use should be relevant to your part size - not too big, not too small) apply the epoxy to your print evenly. It is important not to lather the epoxy on as that will produce an inconsistent and undesirable finish.

  • Leave to dry for up to 12 hours before applying your high-coverage spray paint. Follow the same steps as previous to applying your desired choice of colour paint.

5.0 Additional Resources

5.1 Glueing

Superglue (such as BSI adhesives) and 2-part epoxies are the best types of adhesive to use on 3d-printed PLA parts.

5.2 Molding

3d-Printed parts can be used as masters, positives and negatives for castings or molding. Like many casting processes, molding using 3d-printed parts can be a time-consuming process that requires some trial and error until you receive the desired outcome.

5.3 Vacuum Forming

PLA prints can be used as vacuum form molds with relative success and little deformation. Whilst this is an acceptable method to get molds of various objects and forms, it is important to keep in mind that the accuracy of the resulting vacuum formed molds is not very high.

If you are constructing molds for assembly or if you are working with tight tolerances, you will find that vacuum forming will not be the best process to go through. However, it is a good method for getting quick molds of forms and volumes which you can post-process.

5.4 Making your Prints Watertight WIP

Having watertight prints is crucial for particular jobs, specifically where your parts may be immersed in and/or filled with water (such as a planter pot).

If you are unable to achieve a watertight print, we recommend the use of various sealers and coatings to seal parts, edges or corners of your prints that you suspect to not be watertight. One of the methods we would recommend is the use of Waterproof Wood Glues to coat and seal your prints (such as Titebond-3 Wood Glue).

5.5 Referenced Documents

No-Sand Method Instructional Video
3D Printing Post Processing Coating Demonstrations
Post Processing 3D Printed Parts using Epoxy Resin

5.6 3D Hubs Knowledge Base

Quality articles for engineers, designers and makers to learn about the processes of 3D-printing.