There’s no such thing as a universal standard for optimal film thickness. Everything depends upon the part’s application – its environment, its workload, and the coating’s role therein.
There’s also no easy way to play it safe. Defaulting to more film thickness won’t necessarily do a better job of protecting the part; on the other hand, spraying 1 mil at a time will cost you more than you bargained for. Whether the spec calls for 2 mils or 12, one thing is certain: the further you stray from the target you’re given, the more problems you’ll face.
Inappropriate film thickness causes problems in three areas.
Too much film thickness? Brace yourself for runs (liquid) and back ionization (powder). Not enough film thickness, and the substrate will be visible through the coating.
On either end of the spectrum – too thin, or too thick – expect to see orange peel. This is a broad term, encompassing everything from a “tight” grainy texture on the thin end, to “wide,” loose orange peel on the thick end. It plagues liquid and powder coaters alike, and the best way to avoid it is to stick to the spec.
In the auto body industry, where aesthetics reign supreme, finishers tend to layer extremely thin films one over the other until – eventually – they reach the target film thickness with as little orange peel as humanly possible. The results are beautiful, but the coating process is painstaking and expensive.
Kaser, on the other hand, deals primarily with industrial parts. Whether it’s new metal from the fabricator or a well-worn dump trailer, we blast and coat working class parts with long, hard lives ahead of them, and our mission is to provide maximum corrosion protection at a reasonable price. This means building the appropriate film thickness in as few coats as possible without sacrificing aesthetics.
It’s worth noting that anytime you apply a coating over a surface profile (i.e., after blasting), you must both fill the profile and build film thickness over the tooth. If the valleys are full but the peaks are exposed, it won’t be long before you’re seeing rust spots.
Setting the part and any threat of corrosion aside for a moment, consider the film itself. Will it cure thoroughly? Will it be flexible enough to bend with the part if necessary? Will it be resistant to impact?
If the film is too thick, the answer to those questions is likely “no.” Overly-thick powder coating tends to be brittle and prone to chipping. Spray too much liquid coating, and it won’t cure all the way. Problems abound in either scenario.
In short: film thickness is no place to gamble. Don’t leave it up to chance, muscle memory, or even your best judgment – consult the spec, and then measure your results. If you’re too far outside the target range, start over. Anything less is doing your customer a disservice.