Applying engine enamel

• Work between about 16°C and 25°C in dry air. Every maker's window includes that range.
• Thin coats, several of them. Shake the can for longer than feels necessary.
• Every maker has a recoat window. Miss it and you wait days, because fresh solvent wrinkles half-cured paint.
• Dry, handleable and cured are three different states. Only cured paint resists petrol and brake cleaner.
• Depending on the brand, the engine can run after a day or must wait nearly a week.

Conditions

Each data sheet sets its own working window, and no two match.

The overlap is roughly 16 to 25°C on a dry day. Follow the sheet for the tin in your hand, not the most generous figure you can find. Rust-Oleum adds a caveat the others leave out: its drying times are quoted at 21°C and 50% humidity, and anything cooler takes longer.

Damp air doesn't always slow things down. POR-15's Rust Preventive Coating, which goes under its engine enamel as the prime coat, cures with moisture. It dries quickly in humid weather and can take more than four hours in dry air, the opposite of an ordinary solvent paint.

All of this assumes a clean, dry surface. See preparing an engine for paint.

From an aerosol

Shake for longer than feels necessary. VHT asks for a full minute after the ball starts to move, then ten seconds more for every minute of spraying. Hycote asks for at least two minutes, repeated as you go. Rust-Oleum asks for a minute after the ball rattles, and warns that if it doesn't rattle you shouldn't hit the can to free it.

On distance the sheets differ. Hycote says 20 to 25 cm and VHT says the same in inches. Rust-Oleum says 25 to 40 cm. Whichever you follow, hold the can square to the surface, start moving before you press and keep the distance steady through the stroke.

Coats are thin and there are several of them. VHT wants two light coats and then one medium wet coat. Hycote wants at least two, sprayed sparingly. Rust-Oleum wants two or more light coats a few minutes apart.

The gap between coats is where people come unstuck. Each maker gives a window, and once it closes the next coat has to wait for a full cure. Spray inside the gap and the fresh solvent attacks paint that is skinned over but soft beneath, and the surface wrinkles.

Rust-Oleum's sheet contradicts itself there. Take the longer figure.

When you finish, turn the can upside down and press the nozzle for a few seconds until it sprays clear. Hycote's sheet says so, and it keeps the nozzle usable for next time.

Whether a primer goes underneath depends on the maker. VHT recommends its own engine primer for adhesion and even colour. Hycote says its enamels prime themselves and no primer is to be used. Primers for engine paint has the detail.

From a brush or a gun

On an engine a brush is a perfectly good tool. POR-15's Engine Enamel is about 80% solids and 20% solvent, and the maker says that is why it stays where you put it and dries more slowly than thinner paints.

It handles differently from an aerosol. Stir it, don't shake it. It usually brushes without thinning. Work from the top of the engine down so runs get picked up as you go. POR-15 says the brush marks flow out as the paint dries.

Timing is by feel. The enamel goes over POR-15's prime coat when that coat is dry to the touch but still drags slightly under a finger. The second coat of enamel goes on when the first is dry with no drag, and within 24 hours.

For a gun, POR-15 allows up to 5% of its own reducer through an HVLP set-up. If you already own the equipment it gives the most even finish on a bare block on a stand. For one engine it rarely justifies buying any.

Curing

Dry, handleable and cured are three different states, and only the last gives the paint its properties.

The makers say so themselves. VHT's enamel "only attains its unique properties after correct curing". Hycote says full solvent resistance for its 300°C colours comes after a heat cure at 220°C for 90 minutes. Rust-Oleum gives seven days to full cure in air.

The oven schedules are far apart. VHT asks for 93°C for 20 minutes with the part off the car. Hycote asks for 220°C for 90 minutes. Use the one that belongs to your paint. Small parts such as rocker covers and brackets fit a domestic oven. Expect the smell to linger, so an old oven in the garage is a better choice than the one in the kitchen. A block won't fit, and both VHT sheets say the heat of a running engine finishes the cure on the car.

VHT's own documents disagree about the early stages. The data sheet says touch dry in an hour and handleable in three. The product page says 30 minutes, one hour and 24 hours to dry completely. Rust-Oleum gives 20 minutes to touch and an hour to handle at 21°C. Treat all of these as best cases for a warm, dry day.

When to reassemble and run

You can bolt parts on once the paint is handleable. Go gently, because the film is still soft and a spanner will mark it.

Running the engine is a separate question. POR-15 is the most explicit: wait four to six days before starting it. Rust-Oleum gives seven days to full cure. VHT says three hours to handle and nothing about running, and relies on engine heat to complete the cure.

No sheet describes a painted surface as fit to seal against. Keep paint off gasket faces, as the preparation guide explains.

See every engine enamel in the shop.

Sources

• VHT Engine Enamel product data sheet: conditions, coats, recoat window, bake schedule
• VHT Engine Enamel product page: rating and the second set of drying times
• Hycote Engine Enamels technical data sheet: conditions, coats, heat cure, clearing the nozzle
• Rust-Oleum ATO-65 Engine Enamel technical data: conditions, the contradictory recoat window, cure times
• POR-15 Engine Enamel directions: brushing, thinning for a gun, waiting before the engine runs
• POR-15 Engine Painting Kit instructions: solids content, behaviour in humid air, coat timing