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Activity 002: Surface Finishing and Quality Assessment

Activity ID: U2M4-ACT-002 Duration: 45 minutes Objective: Students will practice sanding, filling, and finishing techniques on a fully cured resin print, then assess the finished part against quality standards. Group Size: 2-3 students per station Materials Cost: ~$5 (sandpaper, primer — parts reused from Activity 001)

Overview

Students take their post-cured print from Activity 001 (or a provided practice piece) and perform progressive sanding to smooth support marks, apply spot filler if needed, and finish with spray primer. They then evaluate the final surface quality under magnification and document the complete post-processing transformation.

Materials & Equipment Needed

  • Fully cured resin print with support marks (from Activity 001)
  • Sandpaper set: 320, 400, 600, 800, 1000, 1500 grit (wet/dry type)
  • Small container of water (for wet sanding)
  • Sanding sticks or foam-backed sanding pads
  • UV-curable spot filler resin (small bottle) and UV flashlight (405nm)
  • Spray primer (grey, filler-type) — instructor-administered in ventilated area
  • Magnifying glass or digital microscope
  • Paper towels and work surface protection
  • Flush cutters and hobby knife (X-Acto #11)
  • Safety glasses
  • Nitrile gloves (for spot filler work)
  • Quality evaluation worksheet
  • Before/after photos (phone camera)

Instructions & Procedure

Phase 1: Pre-Finishing Assessment (5 min) 1. Photograph the post-cured print from 4 angles (front, back, left, right) 2. Under magnification, identify all surface defects: - Support nubs (remaining material from support removal) - Layer lines (if visible) - Surface pitting or holes - Any remaining support tips that were not fully removed 3. Mark each defect with a small dot of removable marker 4. Count and categorize defects on the worksheet: - Support marks: _ (number and average size) - Surface holes/pits: (number) - Other defects: __ (describe)

Phase 2: Support Mark Cleanup (5 min) 5. Using flush cutters, trim any remaining support nubs as close to the surface as possible 6. Use a hobby knife to carefully shave any raised bumps level with the surface 7. Hold the blade at a low angle (10-15°) and scrape gently — do NOT gouge 8. Safety: always cut AWAY from your fingers and body

Phase 3: Progressive Sanding (15 min) 9. Start with 320 grit on the most prominent support marks: - Sand in one direction (not circular) for 30 seconds on each mark - Check progress — the support nub should be level with the surrounding surface - Sand surrounding area to blend the transition 10. Switch to 400 grit: - Sand the same areas plus any rough surface patches - 400 grit removes the 320 grit scratch pattern 11. Switch to 600 grit: - Sand all previously sanded areas - Extend sanding to blend into unsanded areas 12. Switch to 800 grit (wet sanding): - Dip sandpaper in water and sand with light pressure - The water prevents clogging and produces a smoother finish - Sand all surfaces that will be visible 13. Switch to 1000-1500 grit (wet sanding): - Final smoothing pass on all sanded surfaces - The surface should feel smooth to the touch with no visible scratch pattern 14. Rinse the part under running water and pat dry

Phase 4: Filling and Priming (10 min) 15. Inspect for any remaining holes or surface defects that sanding could not remove 16. For small holes/pits: - Put on nitrile gloves - Apply a small drop of UV-curable spot filler resin with a toothpick - Cure with the UV flashlight for 30-60 seconds - Sand flush with 800 grit 17. For spray primer application (instructor supervises, ventilated area only): - Hold the primer can 20-25cm from the part - Apply a light, even coat using slow sweeping motions - Allow 5 minutes drying time - Apply a second light coat - Let dry for 10 minutes before handling 18. The primer coat reveals any remaining surface imperfections — note any that appear

Phase 5: Final Quality Assessment (10 min) 19. Photograph the finished part from the same 4 angles as Phase 1 20. Compare before/after photos side by side 21. Under magnification, evaluate: - Support mark visibility (1-5 scale: 1=invisible, 5=very visible) - Surface smoothness (1-5 scale: 1=rough, 5=glass-smooth) - Detail preservation (1-5 scale: 1=details lost to sanding, 5=all details intact) - Overall finish quality (1-5 scale) 22. Measure a critical dimension with calipers — has sanding affected dimensional accuracy? 23. Complete the quality evaluation scorecard on the worksheet

Discussion Points

  • How much material was removed by sanding? Did it affect fit or dimensions?
  • Which grit transition produced the most noticeable improvement in surface quality?
  • Was wet sanding noticeably different from dry sanding? In what ways?
  • How do you balance aggressive sanding (removing defects) with preserving detail (not over-sanding)?

Expected Outcomes

  • Support marks should be reduced from clearly visible (rating 4-5) to barely visible (rating 1-2)
  • Surface smoothness should improve from 2-3 to 4-5 after progressive sanding
  • Detail preservation should remain at 4-5 if sanding was performed carefully
  • Primer coat should produce a uniform, professional appearance

Assessment Rubric

Criterion Excellent (5) Proficient (3) Needs Improvement (1)
Sanding Technique Progressive grits followed correctly, wet sanding performed, even pressure Correct grit sequence with minor technique issues Skipped grits, excessive pressure, or uneven sanding
Defect Removal All support marks invisible after finishing Most marks reduced, some still faintly visible Marks still clearly visible or new damage from sanding
Detail Preservation All fine details intact, no over-sanding Minor detail loss in 1-2 areas Significant detail loss from aggressive sanding
Documentation Complete before/after comparison with rated evaluations and dimensional check Photos and ratings present but incomplete Missing photos, ratings, or measurements

Safety Considerations

  • Wear safety glasses during all sanding — fine resin dust can irritate eyes
  • Resin sanding dust should not be inhaled — work in a ventilated area or wear a dust mask
  • Hobby knife: always cut away from yourself; retract blade when not in use
  • UV spot filler resin: wear nitrile gloves; UV flashlight: do not shine into eyes
  • Spray primer: use only in a well-ventilated area or spray booth; avoid inhalation; flammable
  • Collect all sanding dust and dispose as solid waste (it is fully cured resin, non-hazardous)

Last Updated: 2026-03-19