Powder-Coat Painting


Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between a conventional liquid paint and a powder coating is that the powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The coating is typically applied electrostatically and is then cured under heat to allow it to flow and form a "skin". The powder may be a thermoplastic or a thermoset polymer. It is usually used to create a hard finish that is tougher than conventional paint. Powder coating is mainly used for coating of metals, such as household appliances, aluminium extrusions, drum hardware, and automobile and bicycle parts. Newer technologies allow other materials, such as MDF (medium-density fibreboard), to be powder coated using different methods.

I wanted to put a more durable coating on my bird feeders than the Rustoleum that I had been using. Packing and unpacking them for craft shows had gotten them scratched and I was ashamed to put them up for sale as my work. So I started looking around at various finishes and finally settled on powder-coat paints. They are available in a wide variety of colors, textures, and finishes (glossy, matte, semi-matte, etc.). The problem is that they require special equipment to apply and then cure them. I started out at my neighborhood Harbor Freight, as usual. They have a basic powder-coat gun system for around $65. Then you get one of your trusty 25% off coupons and you start getting down to around $50. They also sell some powder-coat paints for under $6. Like everything, they aren't the world's best quality, but certainly beats laying out hundreds of dollars only to discover that it wasn't one of your better ideas after all.

Then I needed a spray booth. I borrowed one from my wife's pottery business that I'd built for her. It's basically a plastic storage box. Cut a hole in the back and install the cheapest bathroom fan you can find at your local hardware store on the back side of the hole. In front of that I put some furnace filter material. You don't need to get fancy at first, my filter is held in place with duct tape. Stand it up on end and viola!

Powder paints need to be baked (cured technically) so you'll need an oven. You need to be able to get up to around 350°F - 400°F and keep it there for 10-20 minutes. You could use a kitchen stove, but fumes that deposit on the inside rendering it unusable for preparing food ever again; and the oven has to be electric since the fumes can be explosive. Since my wife has one of these minds that can come up with perfectly acceptable ideas that to my IT-oriented mind would't even occur, I enlisted her help. She turned to the local Target ad and found an electric smoker on sale. It stood tall enough to hang painted parts inside from the grates, so I ordered it. When it arrived, I wrapped the bottom grid in aluminum foil to protect the heating elements in the bottom, took the grid that holds the wood chips and water trays, removed the trays, and inverted it into the top rack slot so that the hanging pieces were hanging from as close to the top of the unit as possible. My work area is outside so I didn't have to worry too much about toxic fumes building up within an enclosed area.

So, for just under $200 I had the equipment to begin experimenting with powder-coated paints.

After the first couple of experiments turned out better than I had any right to expect, I branched out by ordering some different paints. Harbor Freight is limited to red, yellow, white, and black. My first choice was a local suppler here in Tucson. At the time they weren't set up for walk in business, though they did have some colors available by the pound. I odered a couple and they arrived in plastic baggies. First thing I had to transfer them to plastic containers with screw lids so they wouldn't rip and pour all over the floor. They worked just fine and looked better than the HF paints.

Unfortunately, I keep perusing the Internet when I can't sleep. Stumbled across AllPowderPaints.com. Fascinated with some of their colors I ordered one of their two color combinations. I took the option of having them shipped in plastic jars instead of bags based on my previous experience. When they arrived I rushed out to try them out. The first coat turned out a beautiful transparent blue. Then I put on the next coat (silver). What a disaster! It came out shiny grayish silver with hints of purple underneath. Then I tried one with just the silver and it turned out kind of neat; textured silver with black low-lights throughout. I went back to just the blue, but overfired it. I took an angle grinder with a flap disk and ground off the paint and tried again. This time I cured it for only about 10 minutes and it turned out fantastic! The scoring marks from the grinding added some interesting effects since the paint is basically transparent.

I had a lot of trouble getting consistent results applying the two layer powders. It eased somewhat when I switched to a dual voltage gun, keeping it at the higher setting for both coats.

A long time later ... Polyester TGIC

Polyester triglycidyl isocyanurate (TGIC) powders are one of two polyester powder coatings being used commercially (the other is urethane polyester). TGIC powders have very good adhesion characteristics, corrosion resistance, and exterior durability. They can typically be cured at lower temperatures than urethanes and have shorter cure cycles. They also provide good edge coverage and tough, thick films.

Since switching to the PTGIC powders I've had much greater success. Basically, I just put the coated pieces in my oven, close the door, turn the heat all the way up, and come back in around 25 minutes. No more problems with "outgassing", incomplete curing, or uneven colors.

What are the problems with Powder Coating over Galvanizing? http://www.powdercoatingonline.com/html/PowderCoatingoverGalvanizing.html

Hot dip galvanized surfaces have been acknowledged as difficult to powder coat since the technology was first developed in the 1960's. Industrial Galvanizers commenced research in this area in 1986 which culminated in the construction of a purpose built powder coating plant at its Hexham (NSW) site in 1988 in conjunction with its galvanizing operations there. The three main problem areas associated with the powder coating of hot dip galvanized steel products are:

Industrial Galvanizers investigations into these problems, in conjunction with major polyester powder suppliers, resulted in the cause of these problems being better understood, and polyester powder chemistry and plant procedures and design to be modified to allow a consistent, quality assured polyester powder coated hot dip galvanized product to be supplied.

The problems associated with powder coating over hot dip galvanizing have not changed and inquiries are regularly received from throughout Australia asking advice following the failure of powder coatings over hot dip galvanized products.

  1. Pinholing: Pinholing is caused by the formation of small gas bubbles in the polyester coating during the stoving/curing cycle. These bubbles form small craters on the surface and are unsightly. They also produce holidays in the coating that reduce its long term durability, particularly in aggressive (marine) environments. The main reason for pin holing appears to be that the discrete polyester resin particles in contact with the galvanized steel surface do not fuse at the same time as those on the surface of the polyester powder film, because of the mass of the galvanized steel * 1 , and the time taken for it to come up to fusion temperature. Specially formulated resins with 'degassing' agents have been developed to alleviate this problem by delaying the onset of fusion of the powder. Pre-heating the work in a pre-heat oven prior to powder application allows heavier hot dip galvanized sections to be powdercoated and deal with the problem of pin holing when used in conjunction with 'degassing' grades of polyester powder.
    2. * 1 Note : Hot dip galvanized items tend to be of heavier section thickness than other steel items, typically sheet steel, that are powder coated. These items thus take longer to reach oven temperature because of their greater mass.
  2. Poor adhesion: The final stage in the hot dip galvanizing process involves water quenching of the work, frequently in a weak sodium dichromate solution. This process cools the work so that it can be handled and passivates the surface of the galvanized coating to prevent early oxidation of the surface. The presence of a passivating film on the surface of the galvanized coating will interfere with the zinc phosphate or iron phosphate pretreatment, and in many cases, render these pre-treatments ineffective. It is essential that hot dip galvanized items are not quenched * 2 after galvanizing. This ensures that the zinc surface is in a highly reactive state to accept the pretreatment applied in the powdercoating process.
    * 2 Note : It is equally important that the unquenched hot dip galvanized surface is kept clean and dry prior to powder coating. If wet with rain or dew, it will rapidly oxidise and again cause coating adhesion and quality problems.
  3. Incomplete curing: Polyester powders are thermosetting resins that cross-link to their final organic form by being maintained at a temperature (typically 180 o C), for about 10 minutes. Curing ovens are designed to provide this time at temperature combination. With hot dip galvanized items, with their heavier section thickness, it is necessary to ensure that sufficient stoving time is allowed to meet the curing specifications. Pre-heating of the heavier work will assist in accelerating the curing process in the curing oven.
SPECIFICATION FOR POWDER COATING OVER HOT DIP GALVANIZING

The following specification is recommended:

  1. Hot dip galvanize and do not water or chromate quench
  2. Remove all drainage spikes and surface defects
  3. Powdercoat within 12 hours of galvanizing.
  4. Do not get surfaces wet.
  5. Do not leave outside
  6. Keep the surface clean.
  7. Do not transport uncovered loads.
  8. Diesel fumes will contaminate surface
  9. If surface contamination has occurred or is suspected, clean surface with proprietary solvent/detergent designed for pre-cleaning prior to powdercoating
  10. Use zinc phosphate pretreatment if highest adhesion is required. Surface must be perfectly clean. Zinc phosphate has no detergent action and will not remove oil or soil.
  11. Use iron phosphate if standard performance is required. Iron phosphate has a slight detergent action and will remove small amounts of surface contamination. Best used for pre-galvanized products
  12. Pre-heat work prior to powder application
  13. Use 'degassing' grade polyester powder only
  14. Check for correct curing by solvent testing. Adjust pre-heat and line speed to ensure full cure.
SUMMARY

Properly applied polyester powdercoating over hot dip galvanizing will give exceptional performance. Many architectural projects done with this process in 1988 remain in excellent conditions today.

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