Is the idiom, "too much of a good thing" correct? Defined as, "too large an amount of a beneficial or useful thing or activity can be harmful or excessive," this rings true.
Water provides an excellent yet extreme example. Humans need water to survive. Yet too much water and a human drowns.
A less dramatic example is cake frosting. The frosting adds a sweet taste to the baked cake. Too much frosting, though, diminishes the quality of the cake.
In the manufacturing world, adding protective layers to a fabricated component is an apt comparison to our frosted cake. Powder coating is more durable than frosting. We've discussed the benefits two-coat top coats provide. How about adding a third coat?
An obvious benefit a third coat presents is protection against corrosion. Adding that extra layer encompasses the whole part as a buffer against the elements.
Or does it?
On the surface - pun intended - there seems to be no downside to adding a third layer to a two-coat top coat. As good as the idea of adding a layer to two-coat sounds, there are several issues to consider.
Issues When Considering Adding to a Two-Coat Top Coat
Here are five problems when adding extra layers to a two-coat top coat.
The first layer of powder insulates and reduces the electrostatic attraction to the metal surface. This problem requires ensuring proper ground.
The second powder coat layer won't usually attract as quickly to the part as does the first resulting in more application time. The second coat must cover the piece entirely and applied with the recommended minimum thickness. Otherwise, the top layer of powers has a rough appearance, like liquid paint overspray.
Using more powder coating doesn't mean more protection. A ruined finished occurs if there is an excess of powder or a thick layer. Two of the most common issues relating to thick layers are orange peel and sagging.
The powder forms tiny pockmarks and uneven texture ruining any reflective finish. Orange peel with powder occurs when powder does not flow because of possible thin film or formulation. Back ionization from the application usually creates small pockmarks.
If the powder weight is too much, the layer sags, just like paint, leaving ripples and dips on the powder-coated surface.
Improper grounding is the usual suspect for thick coats. If the company performing the powder coating doesn't have the right tools to apply a consistent ground properly, they typically use more powder to compensate. Adding more powder is more expensive, and it may not have a smooth finish.
Excess powder can build up around the part's edge, forming a fat rim, called picture framing. This excess material may also experience the orange peel.
Faraday Cage Effect
Static may impede tight internal corner coverage, which may lead to thick streaks, thin patches, and inconsistent buildup.
The Faraday Cage Effect is common, especially with complex parts. Everything from poor grounding to fine powder makes it hard to cover corners.
Nearly every substrate has tiny pockets of moisture below the surface. Once covered, escaping moisture forms bubbles breaking the surface, leaving it vulnerable to oxidation. Bubbly outgassing is usually associated with castings and can be moisture or air pockets. Outgassing often doesn't occur with hot- or cold-rolled steel.
Overcoming Poor Intercoat Adhesion
Any delay in applying the third coat increases the chance of the material getting dirty. If there's a delay, then extra cleaning steps are needed. However, lightly sanding the surface is also recommended.
Do all third coats create these problems? No, they don't. But these coats present issues ordinarily not seen or as severe as they would in a two-coat top coat.
At the outset, considering the cost from the third coat doesn't seem like much. But solving these problems isn't merely expensive; they're costly when adding quality issues and longer lead times to the mix.