Q: I have a cleaner/phosphate that is supposed to treat magnesium, but how should it be handled as far as dry-off and cure temps? I tried a couple of parts this morning, with a low-gloss clear-coat and they came out looking like Desert Storm camouflage. This was cured for 12 minutes at 180°C.
A: Magnesium alloys are a tricky substrate to powder coat unless you know how to do it. Most magnesium-fabricated products are cast, resulting in a certain degree of porosity on its surface. Cleaning the substrate is a great idea, but the cleaners, pre-treatment, and rinses can remain harbored in the pores.
Indeed, even without cleaning, air resides in the pores. As the powder melts and flows, the cleaners and air escape from the pores. Most powders are curing at this point and can’t recover or reseal the holes caused by the volatiles. The result is pinholes, low gloss, and unsightly surface disruptions.
My advice is to continue cleaning as you are but run the parts through a relatively high-temperature dry-off before you apply the powder coating. It’s preferable to coat the parts very soon after they dry off, even while they are still warm so they don’t reabsorb ambient moisture. As for dry-off temperature, 200°C for 10 minutes is a good place to start.
You should also be aware that many powder suppliers offer product lines that are better-suited for porous substrates such as magnesium. It may be best to use one of these with a well-controlled dry-off process.