Steel Buyers Basics: Painted Steel Price Calculator

Typically, painted steel goes through the following processes:
     Cleaning (both sides)
     Pretreatment (both sides) so the primer coat will stick
     Primer coat (both sides)
     Backer coat (usually thin, protects the top coat when the steel is coiled up)
     Finish or top coat

In a modern coil coating process, these steps are all done (at high speed) in one pass through the coil coating line. Some types of coatings require multiple passes through the coating line.

The largest drivers for coil coating costs are the chemicals to clean and pretreat the steel, the primer paint, the backer paint and the topcoat paint. In addition to these costs are the capital costs of the coil coating equipment, the overhead costs (particularly natural gas to run the paint curing furnaces) and labor cost associated with running the equipment. So if paint costs rise by 4 percent, what is a fair increase in the overall cost of coating steel?

Buyers can determine this if they know the amount of paint used per ton of the steel they have painted. In order to figure that out, first determine the total square feet of the steel per ton and then the amount of paint used per square foot of coil. For instance, 24-gauge galvanized steel (I’ll use 0.024” thickness) x 48 wide theoretically has 2,043 square feet of steel per ton.

A gallon of high-end Kynar paint might cost $100 and be applied at an average film thickness of 0.75 mills. However, most of the paint is solvent (perhaps 60 percent) that evaporates during curing, so only 40 percent of the gallon of paint remains to create the film thickness (known as DFT). There are 231 cubic inches in a gallon of liquid, so one gallon of paint (with 40 percent solids remaining after curing) will cover 856 square feet of coil. Thus, it will take 2.39 gallons of paint to cover one ton of coil. 

Painted ft² per gallon = (0.40 * 231 in³/gal) / (0.00075in *144in²/ft²) = 856 ft²/gal

Gallons/ton = 2,043 ft²/ton / 856 ft²/gal = 2.39 gal/ton

At $100 per gallon, the cost for just the topcoat paint (and only the top coat) is $239/ton or $11.94/cwt. A 4 percent per gallon paint increase means that the actual paint cost increase (again just for the single top coat layer) is about $0.48/cwt. While the topcoat price may be the largest factor in the cost of coil coating, there are many other factors including the costs of the primers, pretreatment, backer and all the other overhead costs that go into coil coating. Buyers should estimate all of these costs (using the same method presented for the top coat) before negotiating pricing.

Typically, coaters will ask for increases for themselves whenever the paint suppliers increase their prices. In negotiation, information is power — astute buyers should try to separate out the paint increase from the additional costs for the coil coater. Be aware, each color of paint, each type of paint and each type of primer will have its own cost per gallon, its own percentage of solids remaining after curing and its own applied thickness – that’s what spreadsheets are for.