Post-Processing for Model Railroaders: Sanding, Priming, Painting

A 3D print straight off the build plate is just the beginning. The post-processing steps you take determine whether your part looks like a cheap toy or a professional model. This guide covers everything you need to know.

Understanding Why Post-Processing Matters

Raw 3D prints have telltale signs:

• **Layer lines** (especially FDM)
• **Support marks** and witness lines
• **Shiny plastic appearance**
• **Rough texture** in some areas

Professional-looking results require eliminating these artifacts and adding realistic finishes.

Sanding: The Foundation of Good Finish

Tools You'll Need

• Sandpaper: 120, 220, 400, 600, and optionally 1000 grit
• Sanding sponges or blocks
• Needle files for tight spots
• Tweezers and picks
• Dust mask

FDM Sanding Technique

FDM prints have visible layer lines that require aggressive treatment:

• **Start with 120 grit** (if needed)

- Remove major support marks and blobs

- Work perpendicular to layer lines

- Don't linger—it removes material fast

• **Move to 220 grit**

- Focus on layer lines

- Use circular motions

- Wet sanding reduces dust and clogging

• **Progress through grits**

- 400 grit for smoothing

- 600 grit for final preparation

- Higher grits only for glossy finishes

• **Check your progress**

- Spray with primer to reveal remaining defects

- Sand and re-prime as needed

Resin Sanding Technique

Resin prints need less aggressive sanding:

• **Remove support marks** with hobby knife first

• **Start at 400 grit**—lower grits scratch too easily

• **Wet sand only**—resin dust is hazardous

• **600-1000 grit** for final smoothing

• **Wear a respirator**—resin dust is dangerous

Pro Tips

• **Sand in one direction** on flat surfaces for consistent texture
• **Use sanding sticks** for small parts—more control
• **Rinse frequently** when wet sanding to check progress
• **Don't rush**—good sanding takes time but pays off

Filler and Putty: Dealing with Problem Areas

Sometimes sanding isn't enough. For deep layer lines or gaps:

Options for Filling

Spot Putty (automotive)

• Quick-drying, easy to sand
• Best for small gaps and layer lines
• Apply thin, sand smooth

Bondo Glazing Putty

• Stronger than spot putty
• Good for larger repairs
• Requires more sanding

CA Glue + Baking Soda

• Instant cure
• Very hard—good for structural repairs
• Creates its own filler when combined

XTC-3D (for FDM)

• Self-leveling epoxy coating
• Fills layer lines automatically
• Adds strength but takes 4+ hours to cure

Application Technique

• Clean the surface thoroughly

• Apply filler slightly proud of the surface

• Let cure completely

• Sand flush with surrounding areas

• Prime and check—repeat if needed

Priming: The Critical Step

Primer serves multiple purposes:

• Reveals remaining defects
• Provides paint adhesion
• Unifies the surface color

Primer Selection

Filler Primers (for FDM with layer lines)

• Rust-Oleum Filler Primer
• Tamiya Fine Surface Primer (Gray)
• Build in layers, sand between coats

Standard Primers (for smooth resin or post-sanded FDM)

• Badger Stynylrez (airbrush)
• Vallejo Surface Primer (brush or airbrush)
• Tamiya Fine Surface Primer

Primer Colors

• **Gray:** Best for revealing defects, neutral base
• **White:** Best under bright or light colors
• **Black:** Best for dark colors, shadow effects

Priming Technique

• **Clean the part** with IPA to remove oils and dust

• **Apply thin coats**—multiple light passes beat one heavy coat

• **Keep distance consistent**—8-12 inches for spray cans

• **Check coverage**—rotate and inspect under good light

• **Dry completely**—24 hours before sanding or painting

Painting for Realism

Paint Types for Model Railroad

Acrylic (water-based)

• Easy cleanup, low odor
• Brands: Vallejo, Citadel, AK Interactive
• Thin with water or dedicated thinner

Enamel (solvent-based)

• Durable finish, self-leveling
• Brands: Testors, Humbrol
• Requires mineral spirits for cleanup

Lacquer

• Fast drying, very durable
• Brands: Tamiya, Mr. Color
• Requires spray booth and respirator

Basic Painting Technique

• **Thin your paints**—ratio depends on application method

• **Apply thin coats**—build up gradually

• **Allow proper dry time**—check paint instructions

• **Paint light to dark** generally—easier to cover

• **Work in subassemblies**—easier access

Brush vs Airbrush

Brushing works for:

• Small details
• Touch-ups
• Beginners
• Limited workspace

Airbrushing preferred for:

• Large surfaces
• Smooth gradients
• Consistent coverage
• Professional finish

Color Matching

For prototype-accurate colors:

• Railroad paint charts (Tru-Color, Scalecoat)
• Reference photos—colors change with age
• Color matching services (paint stores)
• Test on scrap first

Weathering: Bringing Prints to Life

New-looking models don't belong on realistic layouts. Weathering adds:

• Age and character
• Visual interest
• Prototype accuracy
• Integration with scenery

Weathering Techniques

Washes

• Thin paint (10:1 thinner to paint)
• Apply liberally, wipe off high points
• Settles in crevices, emphasizes detail
• Use black, brown, or rust colors

Drybrushing

• Load brush, wipe off most paint
• Lightly drag across surface
• Catches raised details
• Great for wood grain, rust, edges

Powders/Pigments

• Apply with brush to dry surface
• Fix with matte medium or hairspray
• Excellent for dust, rust, grime
• Brands: AK Interactive, Vallejo

Airbrushed Weathering

• Subtle fading and grime
• Rust streaks
• Exhaust staining
• Most realistic but requires practice

Weathering Strategy

• **Start subtle**—you can add more but can't easily remove

• **Use references**—photos of real railroad equipment

• **Be consistent**—match weathering levels across your layout

• **Tell a story**—weathering should make sense for the item's use

Final Finishing

Clear Coats

Protect your work and control sheen:

• **Matte:** Best for most model railroad items
• **Satin:** Slight sheen, good for painted metal
• **Gloss:** Under decals, before weathering, for specific effects

Decals

Apply over gloss coat for best adhesion:

• Cut decal close to image

• Soak in water 20-30 seconds

• Slide onto surface

• Position with brush/tweezers

• Apply setting solution

• Seal with clear coat

Final Assembly

• Use appropriate adhesive (CA, plastic cement, epoxy)
• Add glazing for windows
• Install lighting if applicable
• Attach to layout with removable method

Quick Reference: Post-Processing Workflow

• Remove supports

• Sand (120→220→400→600)

• Apply filler if needed

• Prime (multiple thin coats)

• Check for defects (sand/fill/prime again)

• Paint (thin coats, light to dark)

• Add decals (over gloss coat)

• Weather (washes, drybrush, powders)

• Seal (matte clear coat)

• Install on layout

Master these techniques, and your 3D prints will rival commercial products in appearance.