Electric Paint Basics

What is Electric Paint?

Electric Paint (also called conductive paint or e-paint) is a water-based paint containing graphite and carbon particles. When it dries, the particles form a conductive network, allowing electricity to pass through it. It’s a watercolour-like paint that also conducts — not as well as a copper wire, but well enough for the MPR121’s touch sensing.

Application methods

Bare Conductive Electric Paint comes in two forms:

• Pen (10ml) — a syringe with a fine tip, like a thick marker
• Pot (50ml) — jar form, apply with a brush or screen printing

Pen application

1. Shake the pen well (30+ seconds) — the graphite settles
2. Prime by pressing the tip on paper until paint flows freely
3. Apply with consistent, even pressure
4. Overlap strokes slightly for better conductivity

Brush application (from pot)

1. Stir the pot well with a stick
2. Use a stiff-bristled brush
3. Apply in thin, even layers
4. Multiple thin layers are more conductive than one thick layer

Screen printing

Mix the pot paint slightly with water (no more than 10%) to a screen-printing consistency. Use a 77T–90T mesh. Allow full drying between passes.

What surfaces work

Does not work on: oil-painted surfaces, heavily textured surfaces without primer.

Drying time

• Air dry: 20–30 minutes at 21°C / 70°F
• Fan dry: 10 minutes
• Hairdryer (low heat): 5 minutes
• Oven (50°C / 120°F): 5 minutes (for card/paper — no plastic or electronics)

The paint is not fully dry when it looks dry. It may still be slightly wet in thick areas. Test conductivity with a multimeter before connecting to electronics.

Measuring resistance

Use a multimeter on the resistance (Ω) setting to measure your painted trace. Connect the probes at each end:

• Good trace: 1 kΩ–100 kΩ
• Marginal trace: 100 kΩ–1 MΩ (may work for touch sensing but check reliability)
• Too high: > 1 MΩ — repaint with additional layers
• Broken trace: infinite resistance (open circuit) — look for gaps

The MPR121 can detect touch through quite high resistance traces because it’s measuring capacitance change, not current. But traces over a few megaohms become unreliable.

Line width and thickness

Wider and thicker = lower resistance = more reliable:

• Minimum practical width: 3mm for moderate trace lengths
• For traces longer than 20cm: 5mm or more
• For a large electrode area (palm-sized): paint at least 2–3 layers when dry

Connecting the Touch Board to painted traces

Options for connecting crocodile clips to paint:

1. Clip directly — onto a painted area at least 1cm wide
2. Paint over a copper tape strip — paint over a strip of copper tape, clip to the tape
3. Conductive rivet or snap — press a conductive snap through card, paint over it, clip to the snap
4. Solder — Electric Paint can be soldered (briefly, low heat) for permanent connections

For permanent installations, the most reliable connection is to solder a wire to a copper tape patch that the paint overlaps.

Common painting mistakes

Gaps in the trace:

• Use magnification to check for breaks
• Repaint over the gap with a fine brush

Paint cracking when flexed:

• Use multiple thin layers instead of one thick layer
• Don’t flex the surface before the paint is fully cured (24 hours for full cure)

Contamination from wet hands:

• Apply with dry hands; body oils can reduce adhesion
• Let dry completely before handling

Paint lifting from surface:

• Prime glossy surfaces first
• On wood, seal with PVA first

Key takeaways

• Electric Paint is a carbon-graphite water-based paint that conducts electricity when dry
• Shake the pen thoroughly before use; stir the pot
• Multiple thin layers are more reliable than one thick layer
• Full air-dry is 30 minutes; check with a multimeter before connecting
• Target trace resistance: under 1 MΩ for reliable touch detection
• For permanent connections, solder a wire to copper tape and paint over the junction