Polyurethane, Polysiloxane or Fluoropolymer: Choosing a Finish That Holds Color

Wally Bates

Sep 26

2 min read

Once a primer or epoxy is on the steel, something has to protect it from sunlight. On architectural steel, water towers and exposed structures, that finish coat is also what the public sees. There are three main choices.

Water tower with lettering on a finish coat

Aliphatic polyurethane

For decades the standard finish. Good gloss and color retention, applied at a thin film, reasonably priced and familiar to every applicator. Over time it chalks and fades. On a water tower carrying a town's name or a signature building color, that fading is noticed.

Polysiloxane

Longer gloss and color retention than polyurethane, and in some systems it can replace the epoxy intermediate coat, saving a full coat on a large structure. It is a real step up. If the owner's main concern is fading, though, it is not the top of the range.

Fluoropolymer

Fluoropolymer finishes (FEVE resins) are built on the carbon-fluorine bond, the strongest bond in organic chemistry, and UV cannot break it. That is why fluoropolymer finishes hold gloss and color for twenty years or more, where a urethane may be chalking at seven. Current products are field-applied, two-component topcoats that spray much like a urethane over the same epoxy.

The cost picture

On a repaint, the crew, lift equipment, containment and surface preparation cost the same regardless of the finish coat. The topcoat is a small share of the total. If a project already justifies more than a standard polyurethane, moving to fluoropolymer changes the material line, not the labor line, and it extends the time before the next repaint.

What to check in your spec

Define the owner's priority: color retention, gloss retention or coat count.
Compare finishes on expected repaint interval, not only material price.
Confirm compatibility of the finish with the specified intermediate coat.
For branded colors and logos, specify the finish with the longest color retention.

Further reading: Tnemec Technical Bulletin 06-57, Fundamentals of Ultimate Performance Utilizing Air Cured Fluoropolymers.

Talk to a consultant. Atlantic Coating Consultants helps owners, engineers and contractors in PA, NJ, DE, MD, DC, WV and OH with specification writing, on-site condition assessment, training, product recommendations and field support. Call 484-644-5303 or email atlantic@tnemec.com.