There are several instances when powder coating where multiple coats will be involved. Often times, it is a clear coat for added protection and appearance. Sometimes it is required, chrome powders need a clear to protect them from oxidizing. Candy colors need a chrome base to achieve the candy look. Also, multiple colors can be applied onto one part to achieve a two-tone look. This article will help you apply multiple coats of powder the correct way.
Adding a top coat or second coat should not be much more difficult than the base coat or first coat, however adding a third, fourth, fifth coat will depend on the level of powder coating gun you are using. A cheap hobby powder coating gun like an Eastwood, Craftsman or Harbor Freight will struggle much more with multiple coats, specifically powder attraction, than a mid-level or a professional gun will. This is because the grounded metal part that you are spraying is now insulated by the first layer of powder, and each layer of powder after that becomes increasingly more difficult. Using a grounding rod to ground your parts will significantly aid this process but it can only do so much to help with the shortcomings of a hobby powder coating gun. Learn how to setup a grounding rod.
Another way to overcome shooting multiple coats with a cheaper gun is one that I don't recommend unless you absolutely need to, and this is something that you should only do if you are shooting your own parts. This method involves shooting the part hot so that when the powder hits the part, it will slightly melt, causing it to stick. The issue with this method is that it is very easy to spray way too much powder on the part which can lead to an easily chipped part and possibly orange peel. When shooting the part hot, use the lowest temperature that allows the powder to stick to the part, generally 175 to 200 degrees F, and try to avoid spraying excess powder on the part as much as possible.
A powder coating gun feature that aids shooting multiple coats is the ability to adjust the KV setting. A 2nd coat should always be sprayed with reduced KV, generally 50% less than the first coat. For example, if you shoot the first coat at 70 KV, you would then spray the following coats at 35 to 40 KV. It is best to play with the KV settings and find the sweet spot for the parts you are shooting. These adjustable KV settings are only found on the mid-level and professional powder coating guns.
The process differs slightly depending on the powders you are using. When you are spraying multiple coats and a chrome powder is not involved, a partial cure between coats is the best method. This applies to any The advantage of partial curing a powder before shooting the next coat is intercoat adhesion. This means that the two powders are crosslinked and bonded together as well as being bonded to the part. Failure to partially cure the first coat before spraying the second coat can cause the second coat to peel off. This is called delamination. If you think about it, when you are spraying your first coat over a sandblasted base, the powder has a very textured surface to grip onto. However, if you spray a second coat over a high-gloss base coat, the surface is very smooth and the powder has nothing to grab onto. Partial curing the powders between coats allows them to melt together preventing this from happening. Follow step 3a. below if a chrome powder IS NOT being used.
When you are spraying multiple coats with a chrome type powder involved, you must always fully cure the chrome powder before spraying the next coat. It doesn't matter whether the next coat is a clear coat, candy coat, or anything else, the chrome must be fully cured in the oven before moving on. Unfortunately, this increases the chances of delamination occurring, but it is a risk that must be taken if you want the correct appearance when finished. A chrome powder that is not fully cured can appear to have a veiny cracked appearance when top coated as pictured on the right. Follow steps 3b. below if a chrome powder IS being used.
Step by Step Instructions for Multiple Coats
(no chrome powder involved)
1. Prepare your part just as you usually would. How to prep for powder coating.
2. Spray the first coat of powder just as you usually would. How to properly spray powder coat.
3. Insert part in oven. If any masking was used, it is best to pull the part from the oven after the part reaches 200 degrees and remove any masking. Then insert the part back in the oven and complete the partial cure. Removing the masking at this time gives cleaner masking lines, read more about it here.
3a. Cure the part in the oven at the required temperature but for only 60% of the required cure time. For example, if the powder calls for a cure time of 10 minutes, only cure it for 6 minutes. It is very important that you only start your timer after the entire part itself has reached the required temp, confirm part temperature with an Infrared Thermometer.
3b. (Chrome powder only) Do a full cure of the chrome powder in the oven.I even allow the part 3 extra minutes in the oven just to be 100% that the chrome powder is fully cured. It is very important that you only start your timer after the entire part itself has reached the required temp, confirm part temperature with an Infrared Thermometer.
4. Using clean gloves, remove part from oven and allow it to cool to room temperature. Reapply masking if necessary. Any handling of the part with bare hands can cause issues in the following coat. If you use air to blow off the part at this time, it can cause a static charge that causes the second coat to adhere in odd patterns. If air must be used to blow off the part, or if you wipe the part down, you can disssipate this static charge by heating up the part in the oven for a couple of minutes at 150 degrees F. I try to avoid any need to blow off or wipe the part in between coats.
6. Reduce the KV on your powder coating gun by about 50% if possible and spray the second coat.
7. If the second coat is the final coat of powder you will be applying to the part, then it is time to put it in the oven and do a full-cure following the second powders curing instructions. Again, remove any masking after the part reaches 200 degrees for best results. If you plan to spray additional coats,do another partial cure and repeat steps 4 through 7 until you reach your final coat and then do a full cure following instructions for the last powder applied.
Mulitple Colors on the Same Part (Two Tone)
If you are shooting a two-tone part with multiple colors, the instructions above also apply. The only difference is how you apply the masking. The order in which you shoot the colors really depends on how easy it is to mask one area as opposed to the other. Raised areas are much easier to mask off than recessed areas. The only definitive rule to keep in mind is that red should always be shot last if possible as it has a tendency to bleed through lighter colors, especially white. I will show some examples and how they would be most easily powder coated. It is very important to practice your masking skills. I recommend reading Masking and Masking Part II for some tips.
For the wheels pictured below, the easiest method to coat these would be to shoot the entire wheel in the metallic silver color and do a partial cure. Then mask off the lip, face of the spokes, barrel, and backside of the wheel. Then you would shoot the unmasked areas red. For best results, do another partial cure, and then clear coat the entire wheel. The clear coat helps to smooth any transition lines. When doing two-tones, it is very important to remove any masking well before the full cure is complete, otherwise the edges transition between the two colors will not be a nice, clean, straight line.
To do the two tone valve cover below, the easiest method would be to spray the entire valve cover in the gold vein powder coat and do a partial cure. Then spray the entire valve cover in the metallic red and carefully wipe the metallic red from the letters using a damp finger. Then do a partial cure on the metallic red, shoot the entire valve cover with clear and do a full cure.
To powder coat logos as pictured on the Brembo brake caliper below, a vinyl stencil can be used. You would first shoot the entire caliper silver and do a partial cure. Then apply the Brembo vinyl logo and shoot the entire caliper in red, carefully removing the vinyl logo once the caliper reaches 200 degrees in the oven. Then shoot a coat of clear and do a full cure.
If you would like a custom logo or design that would be too difficult to mask free-hand, a custom vinyl stencil is the best way. For more common logos like the Brembo logo above, they can often be ordered online. However, if you want your own custom logos, you can search for a local sign cutting shop and have them cut it out for you. Alternatively, if you are doing lots of custom logos, you can cut out your own stencils using a Vinyl Plotter. The go-to vinyl for powder coating is Oracal 651. It is not specifically rated for high-temperatures, but it removes cleanly at 200 degrees F. A vinyl plotter can also make short work of masking gasket and mating areas on parts. Just put the gasket surface face down on a scanner and cut out the design on the plotter and you have a perfectly cut out vinyl mask.
You can also fade one powder coating color into another. This can be done two different ways, The easiest way is to simply spray the first color right up to the point you your fade to start. Then without curing, change the color in your gun and shoot the other color on the opposite side up to the transition line and then lightly spray onto the other color however far you want the fade to go. When doing it this way, the two powders must be of the same chemistry. Meaning TGIC with TGIC, Acrylic with Acrylic, and Epoxy with Epoxy. If you are using powders of different chemistries, the entire part should be sprayed in the first color, then do a partial cure in the oven and add the second color up until the point where you want the fade and then cure. Clear can be applied using each method and it will help the appearance of the fade.
Following this information and with a little bit of practice, you should be able to apply multiple coats of powder in any configuration you can think of.
If you have any questions, feel free to leave a comment. Also check out a new section on this site: Recommended Powder Coating Supplies. It has a lot of quality tools that will help you out in your powder coating journey.