Identify Contaminant SourceWe sand aluminum die-cast parts for recoating when they have specks (dirt, metal, dust, etc.) using 120-grit sandpaper on an orbit sander. After recoating the parts, we often have more little specks and other defects and a wavy pattern in the coating. Does this happen because of the sandpaper or because we sand all the way to the aluminum and cause it to outgas?
The first item of business here is to identify which contaminants are affecting the castings and then find the source of the contaminants. After identifying the contaminant source, it will be easier to provide a solution to this dilemma. You mentioned dirt, metal and dust as being sources of these specks in the coating, but are they all present on each reworked part? Although it may appear to be a speck or dirt, if it is not clearly visible below the coating, try to cut around one of the specks with a razor blade and look at the substrate side of the coating with either a jeweler's loupe or a 10X magnifying glass. Based on your observations under magnification, it may be possible to isolate the type of contaminant and help you establish a baseline for further investigation into the root cause of contamination. Quite possibly, you may be able to determine if the substrate is blemished or if the culprit is a metal chip, dirt or dust. This would be a good starting point.
You didn't mention if you were cleaning the parts prior to recoating them. The more involved your cleaning process is, the better off you will be. If you are only doing a solvent-wipe, are you using a clean white cloth for each casting? If these castings are run through a multiple stage pretreatment, is the cleaner stage set above 185°F? Some mold release agents may remain embedded within the pores and on the surface of the casting if the cleaner stage temperature is too low. The residual mold-release contaminants may then outgas during the cure cycle.
Additionally, you didn't mention what type of coating you were using for these castings. Urethane coatings are prone to outgassing and would not be the ideal choice for cast aluminum parts. There are also additives available on the market that can be directly mixed with powder coating to reduce the effect of outgassing, if this is one of your issues.
The wavy-pattern in your reworked parts could be due to an over-aggressive sanding process, in which the actual surface profile of the substrate is unintentionally changed. The sanding profile should be of a uniform etch (anchor profile) not to exceed a maximum of 3.5 mils. Therefore, if your casting has a 3 mil etch and the powder is specified to be applied at 2 mils, then you will need to apply a minimum of 5 mils of powder. 3 mils of powder will only reach the peak of the etch, while the 2 additional mils will provide the specified coverage.
120-grit sandpaper on an orbit sander may remove the coating, but it may be causing more problems than it solves. Is the sander dedicated to only the aluminum castings in question? You obviously risk cross-contamination if you use the sander for other materials and purposes. Another potential problem might be that the abrading media - such as the sandpaper's binder material - is being embedded into the part. At a minimum, consider switching to a finer grit of sandpaper to reduce the appearance of waves and swirls in the cured topcoat. Once the contaminants and the coating are removed after abrading, be sure to re-clean the castings.
When reworking any part, it is best to try to remove the coating from the entire surface if possible. The first coat may reduce grounding by insulating the casting, which would cause premature electrostatic rejection of the second coat. This could also be another possible cause for the appearance problems you are experiencing.
By sanding the substrate with an orbit sander during coating removal, you are introducing locally elevated temperature, which could change the conductivity in the area of interest. This could, in theory, detrimentally affect the collection of charged powder particles by the substrate, possibly giving the appearance of waves.
Again, I emphasize that the first step in resolving the speck issue is to identify the specific contaminants and eliminate their source.