Full Paint Prep Guide: Sanding, High Build Primer & Spray Painting Tips for Scale Models

Introduction

A flawless paint finish on your scale model starts long before you pick up a spray can. Proper surface preparation is the difference between a show-quality result and a finish full of imperfections. This guide walks you through the complete paint prep process — from sanding to high build primer to final spray painting tips.

Step 1: Sanding — Getting the Surface Ready

Sanding removes print lines and surface imperfections, and gives the primer something to grip onto.

Sandpaper Grits to Use

• 80–120 grit — Aggressive shaping and removing large imperfections or filler. Use sparingly.
• 180–240 grit — General surface prep and smoothing after filler work.
• 320–400 grit — Pre-primer sanding to remove scratches from coarser grits.
• 600–800 grit — Wet sanding between primer coats to knock back dust nibs
• 1000–2000 grit — Final wet sanding before colour coats or clear coat.

Sanding Tips

• Always sand in a consistent direction or use circular motions — avoid random scratching.
• Use a sanding block on flat panels to avoid finger dips and uneven pressure.
• Wet sand with water and a drop of dish soap to keep the paper cutting cleanly and reduce clogging.
• Work through the grits progressively — don't skip too many steps or you'll chase scratches forever.
• After sanding, wipe the surface down with a tack cloth or isopropyl alcohol (IPA) to remove all dust and oils.

Step 2: High Build Primer

High build primer (also called filler primer or sandable primer) is a thick, filling primer designed to fill minor scratches, pinholes, and surface texture. It's an essential step before your colour coats.

Why Use High Build Primer?

• Fills 180–400 grit sanding scratches.
• Reveals low spots, pinholes, and surface defects you may have missed.
• Provides a uniform base for colour coats to adhere to.
• Sandable to a smooth finish.

Applying High Build Primer

1. Clean the surface — Wipe down with IPA or a panel wipe product. Any grease or oil will cause fisheye and adhesion failure.
2. Shake the can thoroughly — At least 2 minutes after the mixing ball starts rattling. Warm the can in warm (not hot) water for better atomisation in cold weather.
3. Apply light tack coats first — Mist on a very light first coat and let it flash off for 5–10 minutes. This helps the heavier coats adhere without running.
4. Apply 2–3 medium coats — Keep the can 20–30 cm from the surface, using smooth, overlapping passes. Allow each coat to flash off before applying the next.
5. Let it cure — Allow the primer to fully cure before sanding. Check the product's data sheet — typically 30–60 minutes for a light sand, or overnight for a full cure.

Sanding the Primer

• Start with 400 grit to knock back the texture and high spots.
• Move to 600–800 grit wet sanding for a smooth, even surface.
• If you sand through to bare plastic or metal, re-prime those areas before proceeding.
• Finish with a final wipe-down with IPA before moving to colour coats.

Step 3: Spray Painting Tips

With your surface prepped and primed, it's time for colour. Whether you're using rattle cans or an airbrush, these tips will help you get a smooth, even finish.

General Spray Painting Tips

• Temperature and humidity matter — Ideal conditions are 18–25°C with low humidity. Avoid painting in direct sunlight or on very humid days, which can cause blushing or slow drying.
• Always test spray first — Spray onto a piece of cardboard to check the flow and colour before hitting your model.
• Keep moving — Never stop the spray while pointed at the model. Start and end each pass off the edge of the piece.
• Thin coats are your friend — Multiple thin coats are always better than one heavy coat. Heavy coats run, sag, and take forever to dry.
• Maintain consistent distance — 20–30 cm for rattle cans; 10–15 cm for airbrush. Too close causes runs; too far causes dry spray and orange peel.
• Allow flash-off time between coats — Wait until the surface is touch-dry (typically 10–20 minutes) before applying the next coat.

Rattle Can Tips

• Warm the can in warm water before use for better atomisation and flow.
• Shake for at least 2 minutes before use and periodically during use.
• After finishing, invert the can and spray until only clear gas comes out — this clears the nozzle and prevents clogging.
• Replace clogged nozzles — most brands use interchangeable nozzles.

Airbrush Tips

• Thin your paint to a milk-like consistency — too thick causes tip dry and spatter; too thin causes runs.
• Clean your airbrush thoroughly between colours and at the end of every session.
• Adjust your PSI to suit the paint — typically 15–25 PSI for most model paints.
• Practice trigger control — feather the trigger for fine lines and open it up for broader coverage.

Final Checklist Before You Paint

• ✅ Surface sanded through progressive grits
• ✅ All print lines and imperfections filled and sanded
• ✅ Surface cleaned with IPA — no dust, grease, or fingerprints
• ✅ High build primer applied and sanded smooth
• ✅ Final IPA wipe-down done
• ✅ Paint shaken/mixed and tested on cardboard
• ✅ Painting environment is clean, dust-free, and at the right temperature

Follow these steps and you'll be setting yourself up for a finish you'll be proud to display. Happy modelling!