- The coating needs bare, bright metal. It is a thin film and can't bridge scale or old paint.
- It will not bond to stainless. Check what your system is made of before you buy.
- Follow one maker's system all the way through. Their primer rules contradict each other.
- The heat cure is the last stage of application. Build the temperature up in steps.
- Hycote warns that sudden high heat destroys an uncured coating. Don't fit it and go for a drive.
Get to bare metal
Very high temperature coatings are thin films of silicone resin. They can't bridge scale or cover pitting, so they need bare, bright metal and nothing else beneath them.
VHT's FlameProof sheet asks for the surface to be scuffed or sanded until the metal is bright and shiny, then wiped with grease and wax remover. Hycote's Extreme Heat sheet asks for rust, paint and loose material to be removed by wire brush, grit blasting or abrasive cloth, followed by a wipe with thinners or white spirit. POR-15 asks for its Cleaner Degreaser worked into the surface, then Metal Prep to etch the metal and shift scale. On a rusty cast iron manifold, blasting is the only one of those that gets into the texture of the casting.
Read two warnings before you buy anything. VHT says FlameProof will not bond well to stainless pipes, and that aluminised and galvanised pipes must be blasted first. Many replacement exhausts for classic cars are stainless, so check the supplier's description for yours. A magnet won't settle it, because some grades of exhaust stainless are magnetic. Hycote says not to use its Extreme Heat paints on anything that can't be heat cured afterwards, or that won't get hot enough in use to cure. If you can't cure the coating, it will stay soft and wipe off.
On primer the makers contradict each other. VHT specifies its FlameProof Primer. Hycote says no primer is needed. POR-15 says its High Temp goes straight onto the metal and is incompatible with any primer at all. Choose one system and follow it through to the end.
Mask before you start. Hycote suggests newspaper and tape. The general strip and degrease method is in preparing an engine for paint.
Spraying
Hycote gives a working temperature of 15 to 25°C with good ventilation. VHT gives 60 to 95°F. The Celsius figure printed beside the lower limit on VHT's sheet is a misprint: 60°F is 16°C, and the sheet says 30.
Shake hard. VHT wants a full minute after the ball moves freely and ten seconds more for each minute of use. Hycote wants at least two minutes.
The first coat differs by maker. Hycote wants a light mist from 15 to 30 cm, left for 15 to 30 minutes, then a second coat for full cover. VHT wants the can 20 to 25 cm away in even sweeps: two light coats, then one medium wet coat, ten minutes apart and all within an hour. Miss that hour and VHT says to wait seven days before adding more.
POR-15 asks for two thin coats, or three if the part will meet anything corrosive, each within one to 24 hours of the last. After 24 hours, scuff the previous coat with 320 grit or the next won't stick.
One VHT can covers about 12 to 14 square feet, or a little over a square metre, which is plenty for a manifold and downpipe. It is touch dry in an hour and can be handled in three. The finish is flat. VHT offers a satin clear to go over it, and Hycote describes its own range as matt.
Afterwards, turn the can over and spray until the nozzle runs clear.
The cure
Until it has been heated properly, what you have sprayed is ordinary soft paint. Hycote says its range must be stoved at the stated temperatures to develop its heat resistance. VHT says FlameProof only attains its properties after correct curing.
VHT's schedule with the part off the car starts from a completely dry film. Heat to 121°C for 30 minutes and let it cool for 30. Heat to 204°C for 30 minutes and cool for 30. Then bake for a final 30 minutes. VHT's two documents disagree on that last step: the data sheet says 315°C and the product page says 343°C. A domestic oven stops at about 250°C, so at home the final stage has to happen on the car.
VHT's schedule on the car, again from a dry film: idle for 10 minutes and cool for 20. Idle for 20 and cool for 20. Then drive normally for 30 minutes and let it cool.
Hycote publishes a table of time against temperature in place of a fixed cycle.
| Temperature | Minimum time |
|---|---|
| 230°C | 8 minutes |
| 220°C | 20 minutes |
| 205°C | 30 minutes |
| 190°C | 50 minutes |
| 160°C | 100 minutes |
In use, Hycote says to build the heat up gradually to maximum and hold it there for an hour. Its warning is direct: sudden high temperatures will destroy the coating.
POR-15 runs in a different order. It wants 24 hours on the surface first, then 15 to 30 minutes above 150°C, and says the coating reaches full hardness once it has cooled again.
Hycote also tells you to ventilate during the cure and avoid breathing the fumes. If you use an oven, use an old one in the garage. The coating smokes as it cures, and the smell stays in the oven.
The first run
Treat the first heat cycle as the final stage of application. If you fit a freshly sprayed manifold and drive straight off, the coating goes from cold to several hundred degrees in minutes. That is the sudden heat Hycote says will destroy it.
Expect some smoke and a sharp smell the first time it gets properly hot. That is the resin finishing its cure, and it passes. Bubbling, flaking or bare patches afterwards mean the film was too thick, the metal wasn't clean, or the heat arrived too fast.
Ratings vary by product and by colour. VHT rates FlameProof to 1,093°C intermittent. Hycote's Extreme Heat range gives 650°C for black, 500°C for blue and 450°C for silver, so the colour you choose decides the rating here too. POR-15 rates its High Temp to about 650°C. What those numbers mean is covered under very high temperature coatings.
See every very high temperature coating in the shop.
Sources
- VHT FlameProof Coating product data sheet: preparation, coats, coverage, three-stage bake
- VHT FlameProof Coating product page: rating, cure schedules, stainless warning
- Hycote Extreme Heat Paint Range technical data sheet: ratings by colour, stoving table, sudden heat warning
- POR-15 High Temp paint directions: direct to metal, recoat window, cure