You may have already seen a powder finish dotted with tiny craters, like pinpricks. The defect has a name: outgassing. It mainly affects two types of parts, cast parts and galvanized steel, and it does not come from the powder.
Where these bubbles come from
A cast part, in aluminum or cast iron, is porous. Its pores hold air, moisture and cleaning residues. In the oven, all of that expands and tries to get out, at the very moment the powder melts and forms its film. The gas then punches through the finish, explains Powder Coated Tough. On galvanized steel, it is the air and water trapped in the zinc layer that expand, according to the American Galvanizers Association.
The pre-bake: getting the gas out beforehand
The classic remedy is to heat the bare part before coating it, hotter than the cure temperature, long enough for the gas to escape. Recommendations vary: about 15 to 35 °C above the cure temperature depending on the source, and about one hour for galvanized steel according to the galvanizers association. The manufacturer PPG states in its FAQ that the time and temperature are found by trial, part by part.
The other levers
There are powders and primers designed to let the gas through: their film stays open longer, then closes. Curing at a lower temperature and a thinner film also help. On a die-cast part, it is better not to sand or polish the surface: Products Finishing explains that the dense skin of the casting acts as a barrier, and that removing it makes outgassing worse.
Let's be frank
No method eliminates outgassing for certain on a very porous part. We reduce it, often to the point of making it invisible, but an old cast part that has absorbed oil for twenty years can resist. If your parts are cast or galvanized, mention it in your quote request: we will take it into account in the preparation.
In summary
- Outgassing comes from the metal: air, moisture and residues trapped in the pores or in the zinc.
- The pre-bake, hotter than the cure, gets the gas out before coating.
- Special powders and primers, a gentler cure and a thinner film help.
- On a very porous part, we reduce the risk but cannot guarantee it is zero.
