Powder Coating Finish Problems, Their Causes & Solutions
Nothing can be more frustrating then when you just finished a powder coating project and you find a fish eye in your finish or come to find little dots all over the surface of your part. Regardless of the cause, imperfections in your powder finish can be a headache and can also cost money. This guide will cover common powder coating finish problems and their typical causes so you can take proper steps to keep the issues from happening again.
Powder Coating Finish Problem 1 – Pin Holes in your powder finish
Pin holes look exactly as they are described, dimples and dots all over the surface of what should be a smooth glass like appearing finish. Typically pin holes occur due to inadequate preparation of the surface prior to applying your powder coating. Common reasons you may get pin hole type problems include not properly wiping the surface, resulting in dirt or other contaminants getting trapped in your powder finish, and the other common reason being that a contaminant was actually embedded in the substrate and was not released prior to applying and baking your powder. The first issue can often be addressed with proper cleaning technique.
To help reduce the chance of pin holes from improper surface cleaning you should consider using a dip tank rather than hand wiping (if your handwiping) as this can help reduce the chance of dragging contaminant across the product surface. Additionally, if your powder coating in a booth without doors you may consider a powder coating booth with filter doors to ensure that your powder coating area is properly closed off from any potential contaminants being pulled into your finish while coating your part. Third, you want to be sure your using a proper cleaning agent. For example, in an automated line, it is common to use specifically balanced water formulas. Prior to choosing a cleaner you want to verify with your powder manufacturer that your cleaning agent is ok to use based on your product substrate, the types of contaminants you will be removing from the surface typically, and the how you will be cleaning your part (dip tank, hand wiped or automated line). Finally, if your part may have embedded oil or other contaminants from the machining process you should consider outgassing the part prior to applying powder. Outgassing is when you place the part in a bake oven at a temperature above the curing temperature of the powder to help ensure any harmful contaminants are forced out of the surface of the product by the action of heat opening the parts pores allowing contaminant to be forced out. Finally, you may consider the potential of back ionization occurring. Back ionization occurs when a powder coat becomes to thick and as a result can cause the surface to become insulated which causes new particles of powder to lay unevenly and can result in dots on your product's surface. You can often see this issue when you apply the powder as small craters in the powders surface. To help reduce issues with back ionization you can shoot your powder at a reduced KV and/or move further away from the part surface while applying your powder coating.
Powder Coating Finish Problem 2 – Finish appears unsmooth
If your finish appears uneven and unsmooth this is usually due to orange peel happening. Orange peel is generally the result of one of two primary problems. First, the surface is overly prepped or not properly clean, or second the powder was not properly applied or not properly cured. A full depth guide on orange peel in powder coating and how to prevent it can be found here however will also cover the highlights now.
Surface preparation problems that can contribute to orange peel
Two main surface preparation problems can cause orange peel. First, your surface profile that was created is to deep. This can often be a problem if your sandblasting to prepare your parts for powder coating. If the profile you created is to deep you may have to apply to much powder to cover the profile which can be difficult to get a smooth coat or if its not deep enough you may not be able to apply a sufficient coat of powder to mask the surface resulting in the surface protruding through the powder. If your profile is to deep that is being created you can try using a different grit sandblast media, this chart will show the typical profiles left by a variety of sandblasting media, you can also try lowering your blasting pressure, or moving faster over the surface to reduce the surface profile. If your profile is to shallow and is protruding through your powder coat you can try applying a bit more powder or increasing the surface profile through additional blasting. To know whether your profile is appropriate, you can check the surface profile prior to applying your powder coating with a tool like the visual comparator, more on checking surface profile here.
Powder Coating Application Problems that can contribute to Orange Peel
If your surface is not under or over prepared and you are still having issues with orange peel you should check to verify how much powder you are applying to the product’s surface. Powder coatings will have a technical data sheet where the amount of powder that should be applied is listed, you want to apply only the amount of powder recommended. Tools like a powder comb can help you ensure you apply enough but not to much powder coating to the product’s surface. You will also want to review the section above on back ionization to make sure that it is not contributing to your orange peel, as back ionization can sometimes result in orange peel.
If your surface has been prepped properly and you have applied the right amount of powder you can still have issues if you do not properly cure the powder on your parts. Your powder coating technical data sheet will have a recommended cure schedule which provides a suggested temperature and time to cure the powder coating at. You want to ensure you allow your powder oven to reach the proper temperature and leave it for the correct amount of time because this will ensure that the powder is able to flow out, creating a smooth surface. If your powder was not heated to the proper temperature and/or left in the oven long enough then you could end up with a orange peel in your powder coatings finish.
Incomplete surface preparation contributing to orange peel
A final potential cause of an orange peel like finish can be due to your product outgassing. Outgassing occurs when contaminants are released as you heat your product up which causes either moisture or contaminant to burst through the surface of your product and through the powder coating finish. To avoid this issue it is usually easiest to preemptively outgas your part by heating it to slightly above your powders curing temperature for about 30 minutes which will allow the contaminant or moisture pockets to be released prior to applying your powder coating.
Powder Coating Application Problem 3 – Fish Eyes
Fish eyes look like penny size or smaller craters in the surface of your powder coated part. Fish eyes in powder coating are most often the result of contamination in your product’s surface. Common sources of contamination include silicone, oil, or water. If you are powder coating over a surface that had paint on it before you want to verify prior to powder coating the surface that the surface has no contaminants on it and that the previous paint doesn’t have any contaminants embedded in it. Additionally, you want to ensure you utilize preparation equipment that has been sufficiently clean and consider outgassing your products prior to applying powder coating, especially if they are cast parts, because this will help ensure any potential contaminants trapped under the products surface are driven out of the product.
Common Powder Coating Problem 4 – Inconsistent Powder Finish Appearances
If you notice your powder finishes have variances in their color from one batch of the same powder color to other a few things can be contributing to this. First, check to see your baking the powder at the same temperature for the same amount of time. Under and over baking can result in dull Gloss finishes and other poor color issues. In general if the finish is smooth and free from holes but color is a problem reviewing the proper bake settings and ensuring they are followed is the best step to take to help eliminate the issue. You should also ensure the powder your working with is from a high quality powder coating manufacturer that has good consistency in their batches.
Ultimately while there are a variety of issues that can happen with your powder coating finish, most are attributed to a few causes which include the following. First, not properly removing any contaminants from the products surface or preparing the surface prior to applying powder. Second, applying to much or to little powder and/or not adjusting your powder coating equipment settings to compensate for potential problems like far a day cage or back ionization. Third, not following your powder coating cure schedule properly resulting in under or over curing of your powder coating read more.