A good method for finishing FDM 3d printed patterns

Discussion in 'Pattern making' started by Rocketman, Jun 12, 2022.

  1. Rocketman

    Rocketman Silver

    I ran across this method on Youtube and gave it a try. It's effective.

    The materials & equipment:

    Bondo Spot Putty
    Cheap Airbrush (harbor freight, $10)
    High build primer
    Good ventilation/PPE

    So the basic method is as follows:

    Work in a well ventilated area free of ignition sources (you are aerosolizing Acetone, albeit in relatively small quantities)
    Sand the 3d print with a coarse sandpaper to remove the high spots
    Fill & sand any major problem areas with regular bondo if necessary
    Dilute Bondo Spot Putty with Acetone, and apply using the airbrush, it dries very quickly
    Sand & reapply to problem areas
    Use optional high-build primer to work less problematic areas

    Here's two links showing the method:

    My experience with this method:


    A rough 3d print, not sure what's up with the stringing but can't be arsed to figure it out right now.
    I gave the whole print a hand-sanding with 180 grit paper to knock down the high areas. The deep layer lines in the gentle top contours were filled with regular bondo and rough sanded.


    Left, after airbrush application of diluted spot putty followed by sanding and then high build primer,
    right is the spot putty as applied. I only needed ~1.5 applications of the airbrush putty per pattern half to fill the layer lines, the rest was done by sanding and high build primer

    I applied a layer of laquer to seal the finish and make it a bit durable. The coreboxes I was able to finish mostly just with the high build primer


    And we have a successful test casting using the pattern. These patterns are masters, and will be replicated in a more durable material called Repro from Freeman MFG for use on a multi-impression matchplate
    Tobho Mott, Tops and Melterskelter like this.
  2. Rocketman

    Rocketman Silver

    Final thoughts:
    Sure as hell beats just sanding away like a madman, the results are excellent, IMHO. I believe I have about 4 hours invested in hand finishing this pattern - and they came out way smoother than any other method I've tried with significantly less time invested. No layer lines visible, pulls cleanly from a sand mold - can't ask for much more than that.
  3. Monty

    Monty Copper

    Stringing is caused by running the nozzle temp too high usually. Also look at your retraction settings. Some slicers allow you to stop extruding a little before the end of a line, finish while retracting, and then wipe back into the part. Then the nozzle moves up in the Z and goes to the next start point. Usually playing around with those settings will fix the stringing.

    Nice patterns!

  4. Tops

    Tops Silver

    Rocketman, what type (PLA, ABS, PETG) filament are you using?
  5. Rocketman

    Rocketman Silver

    3D solutech PLA, it was printing fine the last time I ran it. Suspect it's moisture or one of my bowden tube couplers worked loose again
    Tops likes this.
  6. Tops

    Tops Silver

    Thanks Rocketman. Would the acrylic glazing putty work for adding a little more fillet to a PLA pattern? I have one that could stand a touch up rather than a reprint. Got a tube on order just waiting for delivery.
  7. Rocketman

    Rocketman Silver

    I didn't mess with the spot putty too much as it is intended to be used, it seemed to take much longer to dry when applied in thicker sections. It seems like it would be okay for fillets
    Tops likes this.
  8. Monty

    Monty Copper

    Just a few observations, since I've used these methods to smooth patterns in the past. When working with bigger patterns, one must be careful with anything that has solvents. It's really easy to warp/distort the pattern. Bondo is especially bad, because of the solvents, it gets hot while curing, and then it shrinks! I've even had problems with just plain old primer/filler. That is one reason I only seal my lost PLA patterns with Elmer's glue now. It doesn't do much to fill anything, but it seals nicely and does not shrink or warp the pattern. If you are doing sand casting and need patterns, I would definitely make the print "beefy". Then use a primer sealer first before you start smearing on all the goop. Just my .02.
    Tops likes this.
  9. Melterskelter

    Melterskelter Gold Banner Member

    And, so, using a brush you paint on full-strength Elmer's? One coat does it? It takes an hour or a day to dry? After that you can use more or less anything you want for filling and fairing? Has anyone tried PVA for this sealing purpose?

  10. Monty

    Monty Copper

    Hi Denis,

    There are better things out there than Elmer's glue if you are using regular automotive finish techniques. Some primers are specifically meant to act as a barrier for solvents to prevent bleed through and bubbling.

    Elmer's is a PVA glue (polyvinyl acetate). If you are talking about polyvinyl alcohol, I don't know. I have used it as a mold release but never a seal coat. I use Elmer's as a seal coat for parts to keep liquid investment from seeping into the form and causing inclusion defects. It evaporates at my low burnout temp and doesn't leave any ash. I add a few drops of food coloring so I can see the coat as I put it on the natural/translucent PLA prints. I just paint it on and let it dry. Usually overnight. It does a good job of sealing, and seeping into the pinholes in the print. I does a terrible job smoothing. I may try some PVA wood glue to see if it does any better at filling the layer lines.

  11. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Even though it's used in a lot of releasing procedures and always listed as release agent by manufacturers, I'd say it's really barrier. It's always used in conjunction with release agents on one/both sides. If you ever tried it alone without other release agent, you'd be a believer or just read the typical cautions about repeated use/applications to molds. It sticks quite well to most clean surfaces and tenaciously to itself. It just isn't very strong. I've used it as glue for PS foam.

    It's usually very low viscosity so I wouldn't expect any appreciable filling.

  12. Monty

    Monty Copper

    Thanks Kelly, I never really thought about that. I always used PVAlcohol with paste wax. I guess PVA prevents the resin from seeping into pinholes and the wax is the actual release agent. I was always just following the accepted process for mold release with composites.

  13. Tops

    Tops Silver

    Tried the spot putty and acetone airbrush technique. The coating went on dry, parts look 'velvety', couldn't get it to go wet or run if I wanted to (which if this works would be a plus, I am forever spray painting stuff until it runs...). Wondering if I was shooting too much air and/or not enough juice ? 30-35 PSA (2 bar) in the old needle-less and valveless Badger, 50/50 solvent and putty mix. I am going to leave them sit a while and see how it handles.
  14. Smoking Shoe

    Smoking Shoe Copper Banner Member

    Acetone and MEK flash of very quickly. Try adding some lower vapor pressure solvent to the mix. Toluene, AKA toluol, is available in most hardware stores and hangs around longer to let things flow.
    Tops likes this.
  15. Tops

    Tops Silver

    Thanks Smoking Shoe. I will give a' less hot' solvent add a try.
  16. Tops

    Tops Silver

    I wiped at the first coating off with a dry rag and in some case took off most of it, first picture part on the left is back down to the Bondo.
    I tried using Shoe's suggestion of a slower solvent. I have lots of specs paint thinner and only a little xylene so I tried the specs and acetone 1:1 and that mixed with approx the same amount of putty. It sprayed on a little wetter but still not like a rattle can from the store. The color also seemed to change from more red to pink which made me keep thinking I was was coating too thin. The coating is adhered much better, only a tiny amount came off on the paper towel. I can still smell the spec thinner a little on the parts after sitting overnight. Looking forward to a little sanding and final painting. What to use? Shellac, lacquer, enamel? All three? So many choices... I am liking my concrete-mixer-cardboard-box paint booth but it does not vent or draw air so I will need to start wearing an organic vapor mask when doing this. I should probably try another air brush with actual controls for air and fluid and then the toluene but I think will wait for a future pattern for those trials.
  17. Melterskelter

    Melterskelter Gold Banner Member

    I think you know that my vote, if it counts for anything, is for Rustoleum lacquer. My reasons are that it dries very rapidly---15 mins or so between coats, has a high luster so it shows all the little irregularities of the finish, sands well, and, when waxed as a final treatment, provides a very slick pattern surface that draws very well from sand. When applying it I hold the pattern so that light plays off the surface and judge the amount to apply (to avoid runs) by noticing when the surface starts to look wet. That is good enough for that coat.

    If I am trying to build it up a bit, letting the vehicle flash off for a minute or two after applying a couple thin "dull" coats and then applying the wetting coat, seems to work well. If it is a sunny day, I give the wet coat a few mins to flash off and then put it in the sun. If you put it into the sun too soon the vehicle will evaporate too fast and bubble the paint.

    If the initial surface is not ideal ( e.g. reworking a previously waxed surface), a light sanding and then a coat or two of shellac helps the lacquer subsequently wet out the surface. Silicone sprays are bad news---it takes some work to get ahead of fisheyes if you try to rework surfaces to which they have been applied. I avoid silicone spray usage. Graphite, on the other hand, seems to cause no trouble. I use it a lot.

    Tops likes this.
  18. Monty

    Monty Copper

    My approach to this would have been to liberally clean the surface with a wax remover/prep spray. Multiple times, and use something like a tooth brush to go over the surface wet with wax remover, then wash it a couple times with soap and water. Then sand till smooth, and start with a fresh coat of primer-surfacer. I wouldn't try to re-invent primer surfacer by mixing spot putty and acetone. They make the stuff in a can...it's not that expensive. Sand smooth and feather everything down to about 800 grit. Then give it a good coat of primer sealer. If you are doing a really good job, wet sand to 1200, hard coat with paint. Wet sand again, and finally use a rubbing compound to polish. Good suggestion on the graphite instead of wax for mold release.
    Tops likes this.
  19. Tops

    Tops Silver

    Thanks Denis, thanks Monty. The whole process seemed interesting so I wanted to try is since I seem to end up making most of my patterns from 3D prints.
  20. Melterskelter

    Melterskelter Gold Banner Member

    If you are going to be making patterns fairly often, you might think about a high quality 2-part primer. They are expensive, but oh so nice to use. DavidF recommended Evercoat to me. I bought some and have not regretted it. It just goes on sweetly, cures fast and sands like a dream.

    Last edited: Jun 22, 2022 at 6:14 PM
    Tops likes this.

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