It sounds like you bought a stainless steel heater and then found out that stainless steel isn't compatible with your process conditions due to the risk of corrosion/cracking etc. So now you want to "paint" on a Teflon coating to solve the problem.

This isn't likely an "in house" job, and it's not entirely certain that you can do it on a post-manufactured article like an immersion heater. You can't produce a holiday-free coating of a fluoropolymer which is suitable for hot immersion service by painting on a coating out of a can- it's a process which generally involves the application of material in thin layers with fairly high temperature baking steps in-between coats. Those steps will likely destroy the epoxy potting which is typically used to cap the cold ends of immersion heater elements after the electrical terminals are attached.

The watt density of a teflon-coated 600 W heater would need to be very low or it would need to be used under conditions of excellent exterior heat transfer to have the coating survive in corrosion service for any appreciable period.

The geometry of an immersion heater is also a potential problem, making a uniform coating without voids or holidays very difficult to achieve and to verify. At 600 W, your heater probably has an NPT fitting on it, and coating the exterior of male NPT threads with a corrosion-resistant coating is a failure waiting to happen.

The alternative to coatings which is typically used is to encapsulate individual heating elements with PFA tubing or the like. An example is here:

http://www.omega.ca/pptst_eng/VP_VPT.html

Heaters like this are used in severely corrosive aqueous solutions such as plating baths.