Post-processing without losing the fit
Plan sanding, primer and paint; measure how removal, coating and annealing change dimensions.
From observation to a working decision
Finishing changes both appearance and dimensions. Sanding removes material; primer and paint add it. Annealing may change each axis differently. Protect mating surfaces and measure a test coupon before treating a fitted part.
Try the experiment
Work on a 20 mm coupon that must remain between 19.90 and 20.10 mm wide. Adjust removal and coating per side, then enter an assumed measured shrinkage.
Your task
Use at least 0.05 mm removal per side and 0.02 mm coating per side, then keep the final width within the target band. Shrinkage is an input, not a material prediction.
Simplified teaching model. Dimensions and colours highlight the idea; they are not a printer calibration or measured prediction. Orbit changes your view, not the print orientation.
Outside faces
Sanding removes from both sides; coating adds to both. Mask critical mating faces.
Final outside width
19.543 mm
Keep experimenting: the target is not met yet.
The translucent outline is the original 20 mm coupon. Changes are shown at true relative scale; small differences are easier to read in the numbers.
A practical surface sequence
Remove supports and burrs, test a small area, sand gradually, clean away dust, then use a compatible filler/primer and thin colour coats. Let each product cure according to its label. Check fit before painting and mask holes or mating faces where buildup matters.
Dimension bookkeeping
For two opposite outside faces: width after sanding = original − 2 × removal. This model then applies uniform linear shrinkage and adds 2 × coating. A coated hole gets smaller instead. Real removal and coating vary; measure rather than trusting the slider.
Annealing is a separate experiment
Heat treatment can alter properties and also warp or shrink a print. There is no universal recipe for all PLA or PETG. Use the filament maker instructions and a measured coupon; never assume a percentage from this exercise applies to every axis or part.
Workshop practice
Control sanding dust and use the protection specified for your material and abrasive. Use ventilation and the paint/primer safety instructions. Avoid friction heat that softens the surface. Keep coating and heat-treatment equipment suitable for the products used.
Recognize it in the workshop
Terms to keep
- Primer
- A compatible preparation coat beneath the finish.
- Masking
- Protecting surfaces from unwanted coating.
- Annealing
- Controlled heat treatment that may change properties and dimensions.
Use it on your next print
Print a spare coupon beside a fitted part. Measure before sanding, after sanding and after full coating cure. Record changes separately for X, Y and Z if you test heat treatment.
Check yourself
Pick an answer for each question.
1. A 20 mm block loses 0.10 mm from each opposite face. Its width is…
2. Painting inside a hole usually…
3. Before annealing a precision part, first…
Sources (3)
- Postprocessing step by step · Prusa Research
- Annealing tests and dimensional changes · Prusa Research
- Bardo333 — CC BY-SA 4.0 (reference photograph) · Wikimedia Commons
Figures such as temperatures and drying times are typical ranges. Always check the label on your own spool and your printer manufacturer's guidance.