How to Choose the Right Primer & Undercoat for Different Substrates
When it comes to painting classic vehicles, machinery, or restoration projects, choosing the right primer & undercoat is vital. The substrate you’re painting on – bare metal, previously painted surfaces, aluminium, or composite affects which products you use and in what sequence. Use the guidance below (based on Paintman’s in-house advice and best practices) to pick the the most appropriate primer / undercoat system.
When it comes to painting classic vehicles, machinery, or restoration projects, choosing the right primer and undercoat is vital. The substrate you’re painting on – bare metal, previously painted surfaces, aluminium, or composite affects which products you use and in what sequence. Use the guidance below (based on Paintman’s in-house advice and best practices) to pick the the most appropriate primer / undercoat system.
Primer vs Undercoat – What’s the Difference?
Many people confuse these two terms, but they serve very different purposes:
🎨 Primer
Primers bond to bare metal or substrate surfaces, sealing the material and providing corrosion resistance. They ensure strong adhesion between the surface and later paint layers. For example, Paintman Etch Primer is perfect for bare steel or aluminium where adhesion and rust protection are vital.
🎨 Undercoat
An undercoat builds smoothness and provides a solid, uniform colour base for your topcoats. However, undercoats don’t protect against rust, so always apply them over a primer when working on bare metal. We can do undercoat in aerosols to!
Quick tip: If you’re painting bright colours like yellows or oranges, use a white primer underneath to help your colour “pop.”
Recommended Primers and Undercoats by Surface Type
| Surface | Best Paintman System | Why It Works |
|---|---|---|
| Bare Metal (Steel, Aluminium) | 1. Etch Primer → 2. Filler Primer → 3. Undercoat (optional) → 4. Topcoat | The etch primer chemically bonds to metal for corrosion protection, filler primer levels imperfections, undercoat gives uniformity (is optional), and the coach enamel topcoat provides a hard, deep gloss. |
| Previously Painted Surfaces | 1. Filler Primer → 2. Undercoat (optional) → 3. Topcoat | Sand the existing surface lightly to key it, degrease with Prep Clean and re-prime to ensure adhesion and smoothness. |
| Repaired / Filled Areas | 1. Spot-prime bare patches with Etch Primer → 2. Apply Filler Primer → 3. Undercoat (optional)→ 4. Topcoat | Ensures even surface texture and avoids visible filler marks in the topcoat. |
| Aluminium / Non-Ferrous Metals | 1. Etch Primer → 2. Filler Primer → 3. Undercoat (optional)→ 4. Topcoat | Designed for non-ferrous surfaces; excellent adhesion without aggressive acid primers. |
| GRP / Composite / Plastic (if paintable) | 1. Plastic Adhesion Promoter → 2. Undercoat (optional) → 3. Topcoat | Always clean and lightly sand to remove release agents; confirm plastic compatibility before applying coach enamel. |
Step-by-Step: Building a Proper Paint System
Clean & Degrease: Remove all oils and contaminants with Pre-Painted panel Prep Clean OR Bare Metal Prep Clean.
Etch Prime (if bare metal): Apply a thin, even coat for corrosion protection.
Filler Primer: Builds surface thickness and evens imperfections.
Undercoat (optional): Provides a consistent base colour and texture.
Coach Enamel Topcoat: Hand-mixed Paintman Coach Enamel gives exceptional flow, depth, and gloss.
(Optional) Clear Gloss Finish: For extra depth and durability, use Paintman Clear Gloss.
Common Mistakes to Avoid
❌ Applying undercoat straight to bare metal (no corrosion protection).
❌ Using the wrong thinner or mixing products from different systems.
❌ Painting in cold or humid conditions – always check your Simple Painting Guide for recommended temperatures.
❌ Leaving primer uncoated too long – some primers absorb moisture if not sealed.
Pro Tips from the Paintman Team
- Wet-sand undercoats with P600-800 grit for a smooth finish.
- Always filter primer before spraying to remove particles.
- Match your primer colour to your topcoat shade for best coverage.
- Etch and filler primers require different thinners – check each Technical Data Sheet before mixing.
Ready to Start Painting?
Shop the full range of Primers & Undercoats and view our Painting Guide for complete instructions on achieving a professional finish at home.
