The Different Types of Pretreatments for Powder Coatings
Everybody knows that cleaning and pretreating a surface is a key step in ensuring a fantastic coating job and we all want to put our best work out there – for ourselves and for our customers. At IFS Coatings, we always strive for the best with our coatings and we know how important applying those coatings to a well-prepared substrate is, so let’s look at some of the many ways in which we do that. A good pretreatment process can be easy as 1-2-3.
But, what are pretreatment systems? In short, they are the processes through which a surface is cleaned and prepared to be coated. This process should be familiar to anyone that works with coatings and it benefits both customer and coater. Pretreating creates a surface that the coating can adhere to, whether the coating is liquid or powder. Plus, pretreatments heighten the performance value of a coating, increasing its life and helping to prevent corrosion.
That is the hows and whys of pretreatments, but what about the whos and whens? We will start with the first. So, who should use pretreatments? The answer is everyone. Anyone who wants to dust off their hands at the end of the day knowing they have shipped out a quality product should pretreat their substrates.
Plain and simple, a substrate must be prepped before you apply any sort of coating to the surface. The point of pretreatment is to prepare the surface for a coating – of any sort – and prevent it from degrading down the line.
There are many different ways of pretreating and various types of pretreatment available and often it will depend on the substrate you’re coating, the end use of the part you’re coating or the size of your coating line. There are automatic and manual pretreatment processes, multi-stage and single stage, and some that are better for steel than aluminum. The good news is there is a wealth of information available and some great pretreatment suppliers with the knowledge to help you make the right decision for your line.
So let’s look at the different stages of a great pretreatment system and then the various combinations of these stages that are possible.
We’ll get into more details about these below – but, when should pretreatments be performed?
All good pretreatment processes begin with a basic cleaning process. If nothing else, this is the stage that every job shop should perform.
The Cleaner Stage
Whether you are a new or old hat at prepping surfaces, you know that the cleaner stage is the most important and essential stage in prepping any substrate. Oil, grime, and dirt can ruin even the most carefully applied coatings, affecting the finish and how the product you are applying performs – in the long and short term. You don’t want your time and money wasted, nor do you want a dissatisfied customer breathing down your neck.
Cleaning is what every stage after this relies on, including successful powder application. The following pretreatment stages simply won’t work properly if this isn’t done properly. There are many different types of cleaners with acids and solvents being the most commonly used. Typically these cleaning solutions are sprayed onto the part or the substrate is dipped into a pool of the solution.
Top Tip: Heating the cleaning solution will often cause the cleaning agent to work better. Using the parameters given to you by your pre-treat supplier are going to be key. On top of that, making sure you run the process immediately and don't let the newly cleaned part sit is important. You can't clean it, walk away from it for 10 minutes and come back to it and spray it some more, then let it sit before it dries. You have to build a continuous process based on the product that you're using.
The Rinse Stage
After cleaning comes the rinsing stage.
Getting rid of all of that dirt, grime and likely the occasional dead bug is important, but so is removing the remnants of whatever chemical you used to clean the surface. This is key to keeping the coatings uncontaminated. While these rinses can be done with plain city tap water, using Reverse Osmosis (RO) or Deionized (DI) rinses is preferred. This simply means water that has been treated, removing fluorides, chlorides and all the other stuff added to water these days, that are essentially chemicals that we don’t need getting involved in the coating process
Top Tip: When performing rinses, dry the surface as quickly as possible to avoid flash rust on the newly prepped surface. Remember, any runoff needs to be disposed of carefully.
The Conditioning Stage
Preparing the surface to accept product for the best results possible requires some sort of conditioning agent. Essentially, the conditioner is applied, sets the surface to a certain pH level, and then locks that pH level in. Doing this creates a surface that is ready to accept the next layer of preparation.
The Zinc Phosphate Stage
We’re calling it the zinc phosphate stage, as zinc phosphate is highly regarded as an excellent chemical conversion coating, but it could also be iron phosphate or zirconium, which will also do a good, protective job. What this stage does is, not surprisingly, create a layer of zinc phosphate on the substrate. This is sort of like laying down a protective barrier that will prevent corrosion and oxidization of the metal, giving it a longer life.
Once the conversion coating has been applied, another rinse stage is required, performed just like the previous rinse stage.
Adding any sort of phosphate stage to your pretreatment process means enhancing the performance attributes and quality of your work for the customer. A job done well is a job done right.
Moving on to…
The Sealer Stage
Sealing helps the paint adhere to the surface, which means a higher quality finish on the end product, both initially and down the line.
There are several kinds of sealers, including chrome sealers, non-chrome sealers, and dry-in-place sealers. These allow for a stronger performance with corrosion resistance – an essential quality for any substrates that will be exposed to the elements. It should be noted here that chrome is extremely hazardous and dangerous to work with. Specialist equipment and training are required to work with it. In some states in the US, the use of chrome has been banned.
Top Tip: At the end of this stage, you will want to do another rinse, but it must be an RO or DI rinse – meaning no chlorides or fluorides in the water.
The Dry-Off Oven
The final stage is simply drying the part. It may sound obvious, but it is extremely important. The newly prepped substrate must be dry, otherwise, the powder will not apply correctly. A dry off oven basically quickly removes any moisture left on the part from the last rinse. The longer moisture stays on the part, the more susceptible it is to a layer of ‘flash rust’ forming. Getting the part dry quickly reduces the chance and also slightly heats the part ready for powder application. Clearly, the 8 stage process we just walked through is a major undertaking and an automatic line process, with all the cost, space, water treatment and maintenance requirements that come with it. For large volume lines where consistency of pretreatment, high corrosion performance, and warranties this multi-stage pretreatment process will deliver excellent results.
However not every coater either needs or wants to delve this deeply into pretreatment. Fortunately, the pretreatment suppliers have created a selection of smaller spray systems that also offer great results.
One alternative may be a shorter cycle that follows the following stages
- Spray cleaner
- Phosphate conditioner (this combines the conditioning and zinc phosphate stages into one)
- Dry off
Or a shorter cycle again utilizes the “all in one” approach which uses ‘wand’ style equipment (like the weed killer spray solutions you see in hardware stores) and includes:
- Spray clean/condition/phosphate (the pretreatment supplier provides the solution and equipment to enable this one stage pretreat)
- Dry off
Good for lower volume lines, both these options will still give a good pre-treat performance and still provide significant corrosion performance when compared with non-treated metals. Of course, it’s always important to follow the advice of the pretreatment supplier and again, ensure the process is completed continuously. You can’t walk away mid-process, leave it, and then come back to it. It simply won’t work.
These chemical pretreatment options are usually appropriate for aluminum substrates. Pretreating steel tends to follow mechanical pretreatment.
Shot Blasting as Pretreatment
Good news for those that do not like working with caustic chemicals! Shot blasting is another way to clean a substrate and is perfect for steel or for parts where the size of the part of the line makes chemical pretreatment impossible.
Shot blasting is pretty self-explanatory – the substrate is ‘blasted’ with different types of shot; walnut shells, sand, metal soda ash – you name it, there are many different types of shot!
What sort of shot you use depends on the substrate you are prepping. For example, heavier, aggressive shots can damage aluminum, even warp it. Alternatively, finer, mild shots on cold rolled steel can turn a relatively short job into a much longer one. Your shot supplier will be able to advise you on the best type of shot for your needs.
The key with shot blasting is to aim for a “bright white clean”. This is where the surface has been removed to the extent that a bright, ‘white’ looking surface is revealed. Okay, it’s not actually white; it’s essentially a gleaming surface, but it's best known as the “bright white clean”.
When the part is blasted, a blast profile is created and what helps the powder to adhere to the part.
This is a great way to remove mill scale, rust builds up (rust can occur when the part is just lying around waiting to be coated), oils, dirt, weld splatter and more.
Top Tip: Shot blasting also produces heat. A lot of heat. Be mindful of this, especially if you’re working with thin or aluminum parts. On the other hand, this can make it a good alternative for larger or thicker parts that take longer to heat up.
Shot Plus Primer
An alternative, pretreatment to plain shot blasting is to combine a shot blast with a zinc rich or epoxy primer. Using this method, you clean the substrate with the shot blast and lay down a corrosion resistant layer through the primer that the coating can adhere to.
Primers do a great job of adding a protective layer beneath the top coat. Not only do they prep the surface for the top coat to be applied, but they add a protective barrier which will help with corrosion protection.
Top Tip: Ensure that you clean the substrate absolutely, otherwise any coatings that you apply will be rendered useless.
Take note, zinc rich primer does not apply as easily as a top coat. It doesn’t always fluidize like top coat powders do, but you can overcome this with patience and experience – you may have to adjust your gun settings a little - and maybe a bit of swearing, in time.
From cleaning to conditioning to sealing, there are plenty of ways for you to pretreat a substrate in preparation for coating. Each stage has its own good and bad points. What products and stages you use all depend on what you want to achieve for your customers. The whole point of pretreatment is to prep the substrate to properly accept a coating, keep it in top nick for as long as possible and prevent corrosion – and wash away all manner of dirt and bug guts.
These stages and times are in place for a reason, because if you don't follow them your results get poor fast. For many smaller job shops, the least amount of pretreating possible is preferred due to cost concerns. However, it goes without saying that the more pretreatment you do, the better results you will achieve and this is what will have customers returning to you time and time again. If you can, or if you want the best results humanly possible, then the five stage process this guide walks you through is your best bet.
With all of this information at the tip of your fingers, you are now fully prepared to craft your own pretreatment methods and we advise you get support from your chemical supplier to help set things up.
A Final Top Tip:
Think about it - the cleaner stage is extremely important. It doesn’t matter if you have the best pretreating system in the world if you don’t clean the surface properly it is not going to work. Also, stick to the parameters specific to the products you are using and build a process based off of the product that you are using.
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