A lacquer is a plastic dissolved in a volatile solvent, so that it dries to a thin waterproof film on metal, fabric, wood or wire. Celluloid dissolved in acetone or amyl acetate makes a fast-drying lacquer, but both the solvent and the dried film are flammable, so shellac in alcohol is the safer coating for most jobs and the right choice for electrical work.
Part of Plastics and Polymers
Lacquers and coatings are thin protective films formed by dissolving a resin in a solvent and letting the solvent evaporate.
A Dissolved Plastic Dries To A Film
A lacquer works because a linear plastic can dissolve in the right solvent and re-form as a film when the solvent evaporates. That is why thermoplastics make lacquers and cross-linked thermosets do not: see Chain Structure. Plastics from Scratch lists the uses of a celluloid lacquer: metal (corrosion protection), fabric (waterproofing), wood (moisture barrier) and wire (a thin insulating coating by dipping and drying repeatedly).
The Options
| Coating | Solvent | Strength | Drawback |
|---|---|---|---|
| Celluloid lacquer | Acetone or amyl acetate | Fast drying, waterproof | Solvent and dry film both flammable |
| Shellac | Alcohol | Hard, glossy, good insulator | Melts at 75-80 °C; see Shellac |
| Linseed oil paint | Oil | Extends building life; see Protective Coatings | Slow drying |
| Pitch | Heat | Waterproof; see Pitch and Waterproofing | Dark, sticky |
For waterproofing in particular, see also Waterproofing Methods.
A Conflict In The Overview Page To Resolve
The scratch carrier bans celluloid for electrical insulation because its flammability is disqualifying, and yet it lists a celluloid lacquer coating on wire. Both cannot stand. Follow the ban: do not coat live conductors or windings with celluloid lacquer. Use shellac, which the corpus gives at roughly 30 kV per millimetre, or a Bakelite varnish. See Electrical Insulation.
Method: Build A Lacquer Film
Step 1: Prepare the surface. Clean, dry, free of grease and rust.
Step 2: Dissolve the resin. For shellac, the corpus gives about 200 g of seedlac per litre of alcohol. For celluloid, shredded salvaged pieces dissolve in acetone; keep the solvent away from flame.
Step 3: Apply thin coats by brush, dip or cloth. For wire, dip and dry repeatedly, as the carrier describes.
Step 4: Dry each coat fully before the next, in the open air with ventilation.
Step 5: Check the film for pinholes and tackiness. Thin coats dry harder than one thick coat.
Worked Example: Shellac For A Small Batch
At 200 g of seedlac per litre of alcohol, 250 ml of solution needs 200 × 0.25 = 50 g of seedlac in 250 ml of alcohol. If a thin coat covers about 10 m² per litre (an assumed coverage, not a corpus figure), 250 ml covers 2.5 m². Measure your own coverage.
Common Mistakes
- Thick single coat — traps solvent, runs, stays soft.
- Wet coat on a damp surface — bloom and poor adhesion.
- Spraying solvent lacquer near a flame or lamp.
- Using celluloid lacquer on electrical work.
Flammable solvents and film
Acetone, amyl acetate and alcohol are flammable, and most solvent vapour is heavier than air and collects in low places and pits. No flame, sparks or hot surfaces in or near the work area, and keep ventilation up. Dried celluloid film burns violently, and the dried film should not be heated. Keep solvent containers closed and small.
What’s Next
- Learn next: Electrical Insulation — choosing a coating for a conductor
- Combine with: Celluloid Safety — the hazard behind the advice