Cast Iron Finishing Options for Trench Drain Systems

Trench drain is becoming a bigger part of American hardscape and streetscape projects as North America continues to builds its infrastructure and becomes concerned about water quality and storm water control. Higher aesthetic demands are being made on trench drain grating on streets and sidewalks.  Because of this, it is a common practice to “finish” cast iron grates – to coat or treat them in order to protect the grates’ integrity and look.  This article focuses on five finishing options for Cast Iron grating and their uses in applications from industrial to residential.

Raw Cast Iron (Uncoated)

Oblio iron grates, Iron Age deco grates, raw cast ironCast iron is, by far, the most common metal used to make trench grating because of its relatively low cost and good functionality.  The current standard finish that is supplied with decorative and municipal-highway trench grating is raw.  Raw (uncoated) cast iron is what you get when you don’t coat the casting, leaving it to the elements.  The foundries that manufacture the products each have their own reasoning, but one fact remains true:  foundries just love to sell raw castings!  In fact, because most aren’t set up to offer painting or coating services, it’s sometimes like pulling a hen’s teeth to have a foundry supply anything other than a raw, uncoated iron grate.

Raw cast iron will naturally rust and develop a dark patina, and this thought process leads many municipal-highway foundries to store their product outdoors.  After all, their product will be used on the street and be exposed to lots of water anyway.  By installation time a municipal-highway may have already had three years of exposure to sun and rain.

But that can be a long time coming for a grate:  in some environments, a grate can remain a rusty orange color for years, bleeding the color into the surrounding concrete and creating unattractive stains.  This is pretty important in residential applications where grates are chosen not only for their load rating but for their tidy appearance.  Perhaps a better industry-wide motivating factor toward the move to finished grating, though, is that corrosive environments can actually wear a raw grate away faster.  That creates hazards and replacement costs over time.  Yikes!

Bituminous Coating

Bituminous-Coated-GratesA bituminous coating is the second finish type that I have seen on municipal-highway castings. The first type was… you’ve guessed it! Raw cast iron.   Bituminous coating is a cheap way of “water-proofing” the grating for a few years.  The top surface of the grating will lose the protection in time, but the underside of the grate will remain coated, which will help prevent radical oxidation of the iron. In the past, the casting may have been coated with a light tar coating.  The tar was applied by actually dipping the casting in a vat of tar.  This was usually done at the request of the project’s architect or the government agency involved in specifying the grating.

The bulky ductile iron or cast iron grating made for highway or municipal applications is supplied by foundries such as Neenah, Campbell or East Jordan.

Epoxy Coating

Commercial-Cast-Iron-GratesFrom everything I’ve seen in commercial trench drain, the grates that are supplied by manufacturers like Polycast, MEA, ABT and Zurn have all been painted with black epoxy paint (see below photo).  I can’t remember ever seeing an uncoated cast iron grate supplied by any commercial-industrial trench drain manufacturer.

Below is a photograph of a trench grate painting station in China.  These products are mostly being shipped to Europe but still demonstrate that surface coating is a common occurrence in the commercial and industrial trench drain market worldwide.

One problem with an epoxy paint coating  is that it will eventually wear or chip, exposing the raw cast iron underneath and leading to localized rust formation.  The rate that this reaction will occur depends on the severity of the application but, in comparison to powder coating (see below for more information), epoxy coating seems to be more rugged and durable.  Some paint facilities recommend using a base coat of epoxy paint prior to a powder coating operation.

Powder Coating

Baked-on Oil Finish vs. Powder Coated prPowder coating is a process where “powdered paint” is electro-statically deposited on a metal part and then heated to a temperature that allows the powder to melt, intimately coat the part and harden.  This method of coating is more efficient and safer on the environment than traditional wet paint coating because a much higher amount of the paint adheres to the part.   This coating method is used with decorative trench drain grating for applications that are not going to see heavy vehicle traffic.   If this coating is chipped off by some hard impact, it could lead to an exposed cast iron surface that would then rust. This is a primary reason I only recommend powder coated trench grating for residential and decorative applications.

NDS recently began selling decorative cast iron grating options for their Dura Slope product line.  Their Deco Grate products include five (5) ADA compliant patterns that are available in one of six standard powder coated colors.  You can also purchase these grates without a coating.

Decorative trench grating is a boutique product in a growing marketplace.  And recently, it has been receiving a lot of attention from architects and homeowners alike.  There are a handful of small foundries that offer ornamental trench grating as part of their product line.  Previously, the trench drain grating made by the majority of these foundries did not fit a specific “off-the-shelf” trench drain system.  Today, decorative products are available that fit standard pre-formed trench drain systems and can be supplied with a number of surface finishes, including powder-coating and the popular Baked-on-oil Finish (BooF).

Baked-on-oil Finish (BooF)

The baked-on-oil finish is becoming more popular as a grating surface finish.  To achieve the baked-on-oil finish, castings are immersed in recycled vegetable oil and then heated to 375-450 degrees Fahrenheit.   Once applied, the casting surface will have a chocolate brown color and a slightly shiny texture.   Some foundries begrudgingly offer the service when the customer “demands it;” other foundries make this finish readily available.

There is some debate surrounding the merits of the baked-on oil finish.  One debate seems to focus on whether the emissions given off during the baking process are eco-friendly.  As the oil used in BooF is recycled, there is some environments savings.  However, some question whether the emissions given off as smoke during heat treatment is worthy to the environment given the temporary nature of the coating.

Another discussion concerns the temporary effect of the baked-on oil finish.  If applied properly, the BooF patina seems to last a while and helps the casting age gracefully into the iron oxide coating that it will eventually become.  If the oil coating isn’t thorough or the baking process isn’t complete, an orange iron oxide film (which is so disdained) will appear after one rain.


Iron Age sun grate, weathered sun grate, weathered cast iron, In summary, the choices of cast iron surface treatments include bituminous coating, epoxy paint, powder coating and baked-on oil finish.  All these coatings offer a degree of protection against oxidation and improve the aesthetic of the grate.  A raw, uncoated trench grate will quickly develop a fine orange rust layer on the surface with the on-set of oxidation.  This will eventually pass and stabilize into a dull chocolate brown patina.  Depending on the application of the grate and the desired look being sought, one of the coatings discussed above may be useful.

Trench Drain Systems, a national distributor of trench drain systems and decorative grating, is proud to have brought you this article.  For further information on cast iron grating finishes, leave a comment below, email us or call at 610-638-1221.

3 thoughts on “Cast Iron Finishing Options for Trench Drain Systems

  1. joe keesee @ Consolidated Supply Co. says:

    Looking for Pricing and supplier for RAWCAST IRON w/baked on oil finish.
    60 LF of trench drain w/ grate size 4.9″ x 19.6″ x 3/4″ w/GREEK KEY STYLE
    1-503-906-3291 Thank you

  2. Pingback: » Mini Channel’s Decorative Grating Options

Leave a Reply