- Engine enamel tops out at about 340°C and a manifold runs hotter than that. The cans here start at 650°C.
- The rating is a peak the film survives, not a working temperature, and most of them say intermittent.
- Every one of them is soft until it has been heat cured. The cure is the last stage of application, not an afterthought.
- Stainless pipes rule out VHT and rule in U-POL, by the makers' own sheets.
- Price per 100 ml is shown on the day, from Amazon's figure, and the picks are read against it.
Why engine enamel will not do
The best engine enamels stop at about 340°C. Eastwood rates its ceramic enamel there, Hycote the top of its range at 300°C and VHT its Engine Enamel at 288°C. A cast iron manifold on a road engine passes that in minutes, and a header on a tuned one runs far hotter. Nothing sold as engine enamel is rated for that, and none of the makers says it is. The paint type page sets out the two classes of product side by side.
So the shortlist is very high temperature coatings only: silicone-resin or ceramic-pigment films rated from 650°C upwards. The shop lists every one Amazon UK sells, and these five are the ones the makers' sheets and the price on the day put at the top.
Reading the ratings
The figures run from 650°C to 1,093°C and they are not comparable at face value.
VHT gives FlameProof a range, 1,300°F to 2,000°F, and calls it intermittent. U-POL gives Raptor Flameproof 1,000°C, and its flat black sheet also calls that intermittent. POR-15 gives 1,200°F and converts it to 648.8°C, a precision no manifold will respect. Jenolite and Simoniz give one figure each with no qualifier. An intermittent figure is a peak the film survives for a while. It is not a temperature the paint will sit at all day.
For a manifold on a road car, 650°C is enough and the difference between the picks is in the cure and the surface. For a header on an engine that gets worked, the two coatings rated above 1,000°C are the only ones with headroom. That is why they are first and second.
The cure is the job
Every can here is ordinary soft paint until it has been heated properly. VHT says FlameProof only attains its properties after correct curing. U-POL wants four hours of air drying and then either an hour at 150°C or two half-hour idle runs. POR-15 wants 24 hours on the surface and then 15 to 30 minutes above 150°C. Jenolite says the full cure arrives at 260°C in service.
The schedules differ, and the one to follow is the one for the can in your hand. What they share is the warning against sudden heat: fit a freshly sprayed manifold and drive off, and the film goes from cold to several hundred degrees in minutes, which is exactly what destroys it. Painting manifolds and exhausts draws VHT's bake and the staged idle cycles, and explains what the first run should look like.
Off the car, a domestic oven stops at about 250°C, so VHT's last stage has to happen on the engine. U-POL's 150°C and POR-15's 150°C fit an oven. If you have a bare manifold on the bench and an old oven in the garage, those two are the easier cures.
Stainless, aluminised and galvanised
Many replacement exhausts for classic cars are stainless, and this is where the sheets part company.
VHT says in as many words that FlameProof will not bond well to stainless pipes, and that aluminised and galvanised pipes must be blasted first. U-POL's sheet lists stainless steel, galvanised steel, aluminium and titanium among the surfaces Raptor Flameproof goes straight onto. Neither Jenolite nor Simoniz says either way.
A magnet will not settle what your system is made of, because some grades of exhaust stainless are magnetic. Check the supplier's description. If it is stainless, the second pick is the first pick.
What to check before you buy
Three things, in order.
What the part is. A cast iron manifold or a mild steel downpipe takes any of the five. A stainless system takes U-POL. A part you cannot heat afterwards, because it will never get hot enough in use, takes none of them: the coating will stay soft and wipe off.
Whether you can cure it. Off the car with an oven that reaches 150°C, or on the car with a morning to spare for idle-and-cool cycles. If neither, the job waits.
Whether you need a primer. VHT and U-POL both name their own primer for the full rating. POR-15 forbids one. Jenolite and Simoniz say nothing. Follow the sheet for the can you bought and do not mix makers.
Then the price per 100 ml, which the table shows from Amazon's figure on the day and which moves. The figures above do not.