Ask Yourself These Questions
Before Selecting a High Temperature Protective Coating

When selecting, specifying, or using high temperature coatings, it is important to realize that not all high temperature coatings perform the same. There are many raw materials involved when formulating heat resistant coatings, but the primary constituent is silicone resin. In formulating heat resistant coatings, there are many silicone resins, from many raw suppliers to choose from. The silicone resin a coating manufacturer chooses will effect the finished coating in many ways: 1) heat resistance 2) dry time 3) hardness 4) color stability 5) thermal shock resistance 6) cure 7) compatibility with primers and other topcoats 8) cost 9) corrosion resistance 10) application characteristics and 11) VOC requirements etc.

The characteristics/features of a high temperature coating are also affected by the modification of blending organic resins with silicone resins, e.g.: acrylic, alkyd, etc. In formulating a color stable, silicone, acrylic topcoat to 500°F, (260°C), there are three important considerations: 1) selection of silicone resin 2) selection of acrylic resin and 3) the ratio of silicone to acrylic. All three will greatly affect the above mentioned characteristics of a finished high temperature coating.

More so than with any other type of generic coating category, (epoxy, urethane, zinc, etc) selecting a quality high temperature protective coating will save you money and spare you a headache. Selecting a heat resistant coating to 1000°F for a 300 ft. stack should be dependent upon it performing well . . . . because if it doesn't, having to repaint a 300 ft. stack is a tall order and a costly one.

1) What is the maximum exterior metal temperature of the equipment you wish to protect?

Select a coating which is able to withstand the maximum exterior metal temperature, otherwise the coating will fail. Simple answer but true.

2) Is the equipment which needs to be protected insulated or not?

If the equipment is insulated, keep in mind the coating you select should be resistant to cyclic exposure of boiling water and dry high temperatures. If you already have existing corrosion under insulation, you may need to select a coating which also meets the following criteria: 1) be applied directly to hot operating equipment and 2) have high film build capabilities, (12-18 mils. D.F.T.). These features may be necessary to protect the peaks and valleys of pitted steel which may have occurred by the corrosion.

3) What are the thermal cyclic conditions of the equipment?

There are many generic high temperature materials rated for a particular heat resistance. However, they may not be able to withstand cyclic conditions. Be sure to select a coating system which can withstand thermal cycling. All equipment at one time or another needs to be started up or shut down. Even a piece of equipment having a metal temperature of 700°F, which runs 24/7 and almost never shuts down, is exposed to thermal shock . . . . for example when it rains, the water hitting the 700°F surface causes a shock to the entire coating system.

4) What is the metal temperature during the application? Will the equipment be shut-down (ambient) or in operation (hot)?

Your metal temperature during application will dictate if you are going to use a material which can be applied to ambient steel or use a high temperature coating which may be applied directly to hot operating equipment. There are protective coatings today which may be applied directly to hot steel having a metal temperature of up to 500°F.

5) What type of steel are you coating? Carbon? Austenitic Stainless?

The type of steel you wish to protect will effect your selection of primers and topcoats. For example, coatings used for the prevention of chloride induced external stress corrosion cracking of insulated austenitic stainless steel should be: 1) resistant to boiling water 2) resistant to high temperatures and 3) have a low level of leachable chlorides.

6) Which high temperature primers should I select for use in atmospheric exposure?

Several parameters will dictate which primer to use. These conditions are: 1) heat resistance 2) environment/exposure 3) thermal cycling 4) dry time 5) cure and 6) recoatability. Primers to select from are: 1) modified zinc silicone, 600°F 2) inorganic zinc, 750°F 3) zinc silicone, 1200°F and 4) a high build system, 1000°F. More people in the coating field are using a high build, heat resistant primer for all their atmospheric exposure primer needs from ambient to 1000°F. It eliminates pin point rusting, (notorious for thin film primers), offers better barrier/corrosion protection, (6-8 mils. D.F.T.) and having a low blast profile during surface preparation is no longer critical, due to its high film build capabilities.

7) Has the surface you are coating been previously painted? Is it in good condition? Is there much corrosion?

Determining the condition of your steel will effect whether you: 1) simply overcoat the equipment 2) spot prime and overcoat or 3) remove all old coatings and corrosion and apply the proper high temperature primer and topcoat.

8) What are your dry time parameters?

Some high temperature primers and topcoats take long periods of time to dry. Others remain soft and/or tacky and never truly dry until being exposed to heat. If your time table is short, use materials which will enable you to complete the job in the time allotted. Be sure to select coatings which will dry quickly and hard, since it will aid in reducing any mechanical damage to the system.

9) What are your cure capabilities?

Selecting a coating which does not require a "complex post cure" will make the job simpler and reduce cost and reliability. Selecting a coating which requires no cure makes your project that much easier.

10) What are the Federal, State and local VOC regulations requiring you to use?

Check with your local and state agencies for your VOC regulations in respect to high temperature coatings. The federal regulation is 5.4 lb./gal. (648 g./l.) but many states are now requiring a VOC content of 3.5 lb./gal. (420 g./l.) for high temperature coatings.

11) What are your color requirements for the project?

As a final note, when selecting high temperature resistant primers and topcoats, there are many factors which have to be weighed in the decision-making process. All of the above questions should help that process. Keep in mind each project will have its own set of variables and parameters leading to the selection of varied systems. Select the system that best fits your needs.

For more detailed answers or if you have other questions regarding high temperature protective coatings, please contact us.

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