It’s a product for the foundation of painting and part of the body repairs, but the term “primer” is vague once you consider all the “primers.” In simple terms, it’s meant to promote adhesion of additional products and provide a ground coat foundation. It’s also typically an identifiable medium to dark gray color, but it can also be found in other colors. Most body technicians think of primer surfacer when referring to the subject. Painters may think of primer sealer, and these barely cover what primer is in the whole automotive body repair process after the OEM builds the car. Along with their limitations are on the repair end and what is overlooked by just grouping all these primers together.
Primers: the foundation and corrosion protection
From the factory level when the OEM builds its cars, the whole body of the car may be dipped in a solution of wash primer that is essentially various acids and a metal conversion coating to protect the vehicle from rusting. This is followed by additional coatings such as e-coat primer, along with the rest of the coatings to give the car color. When we repair it and it’s down to bare metal from the accident or repair process, we have exposed bare metal that must be protected against corrosion. This is not limited to the obvious rusting steel can do, as even aluminum will corrode, albeit in a different way, as it oxidizes and can cause material failure that at its worst could compromise safety. So we have some primer category products options we must apply to restore corrosion protect to not cause failure:
Wash primers / etch (acid) primers / metal conversion products
While these may be called "wash," "acid," "etch," or "acid etch," these primers all work similarly from one paint company to the next. They try to mimic the early coatings process at the factory when the whole car was submerged in a solution to convert the metal surfaces. Some of these products are 1K, meaning they’re not mixed with another chemical, so there is not a reaction needed for them to cure. That’s why you often saw these in a dedicated paint gun hanging up in the mixing room ready to spray until the next job that needed protected. Some issues arise from these products being misused and the technical data sheet (TDS) not being read. The most common is body technicians doing their metal repairs and ending with body filler and applying primer to the whole repair area because of the exposed metal. This must be avoided, as the acids will react with the body fillers. A similar situation is the paint department first applies an etch primer to large metal areas exposed, then the body department applies body filler on top of it. This is a situation with incompatible products. These acid primers must be separated with a different layer of a different type of primer such as primer sealer (more on that below) to not cause a failure of products.
Some companies, such as AkzoNobel, have even developed wipes that resemble a household cleaning disinfectant wipe. These can be easily used on cut-through metal areas or large areas to convert the metal surface for corrosion protection. They are great for last-minute catches in the booth when you miss seeing some exposed metal areas and are ready to do the rest of the priming so you can quickly protect the metal.
Epoxy primer
This is viewed by many to be the true high-quality choice in restorations to protect metal from corrosion, and some OEMs even require this to warranty the repair before topcoating with any body repair or paint materials. But there can be a tradeoff in productivity, as epoxies can sometimes take a while to dry. So even when mixing a small amount to apply to, say, new spot welds prior to seam-sealing, the long curing time is sometimes a complaint. The payoff is this will not react with repair materials under or above such as body filler, and it can combine the steps of corrosion protection and sealing repairs from additional repair layers.
Hybrids
While not typical, some of the more common primers in a shop, surfacers and sealers, are sometimes manufactured to be combined to act and spray the same way but have the chemical properties of an epoxy to also provide corrosion protection.
Making “e-coat” and other 1K primer products
Some primers we may have at our disposal in a shop or want to use are 1K aerosols. The 1K means one part, with no hardener or activator mixed in the can. But there actually are 2k aerosols that are the exact same as what you’d mix for your paint gun but in an aerosol form. These 1K primers may not be that durable in general, but especially if used over bare metal they have absolutely no corrosion protection. Any additional body repair materials or paint coatings are at risk of failing from the bottom up, and it’s not as durable of a repair. Consequently, it will not live up to attempting to replicate to pre-accident condition. They should not be relied on if attempting to protect the metal only. Some products are color-matched to the factory e-coat. While they are a convenient way to match colors, again they do not restore corrosion protection of what wasn’t already achieved. They also need to be topcoated with clearcoat to protect from elements, especially UV rays if exposed. Technicians think they’re recreating the “e-coat,” which is not possible at the repair level. True e-coat, electrodeposition coating, is made with a negative electrical charge on the part and a positive electrical charge on the chemical coating being applied, ensuring even application to nooks and crannies. This is performed with the whole body or individual parts submerged using specialty equipment that isn’t available at the repair shop level. These aerosol products’ function are only to quickly match the factory e-coat’s appearance.
Primer surfacer
As previously noted, surfacer is the product most commonly associated with the word primer, particularly for body department repairs. Its main job is to help transition the dent and surface coatings that are scratched from sanding, welding, and other body repairs in preparation for additional paint coatings. It is not meant to fill. Most of these products, even when applied to the highest ACCEPTABLE mil thickness by the manufacturer’s technical data sheet (TDS), are not enough to truly “fill.” Typically, this product has zero corrosion protection and is meant to only help transition the various repair spots. So, covering raw metal from the body repair process needs to be addressed with one of the anti-corrosion primers. Even though the metal, body filler, and other products will need to be straight, after applying this over all those transitions it will still need a final blocking to ensure straightness of the whole repair area.
Seal the deal: Primer sealer and ready for color
This is the other common “primer” people think of. This is meant for closing up many repair situations that are ready for true painting of colors. These do not have much material mil thickness build and are meant for separating or assisting with color evenness. Some examples are sealing off products from each other, such as acid primers from other repair materials, basecoat color repairs, and covering anything else needed to continue with color coatings. These are available in different colors, including white, gray, black, and shaded in between, to a range of colors to help with easier hiding of the topcoat. It is also sometimes required for very unique or difficult colors, as prescribed within the paint manufacturer’s formula, or you’ll never get the proper look for the color. These usually do not have any corrosion protection ability unless they are a hybrid.
Waterborne primers and UV primers
While most primers, particularly the surfacers and sealers we commonly use in a shop, are hardened by the chemical reaction of mixing products, there are some that use other methods for additional speed or, in some areas, environmental regulations. Ultraviolet (UV) primers can cure only by using special UV lights ranging from handheld to large lights. Sometimes, if allowed by the TDS, they can even be set by the sun’s UV rays by putting the repair part or car in the sun. They rapidly set – almost instantly – by the UV rays instead of waiting for a chemical curing process. The downside can be ensuring that the primer actually is cured by the UV method of choice by the technician. If it’s not, it will never cure and it can fail if all the layers aren’t cured. So this can have limitations if you can’t get the light in all the areas you spray. There are also primers that use waterborne technology, similar to that ofsome basecoat systems as the carrier of the liquid, instead of solvent chemicals. They evaporate the liquid out over time or through forced air equipment, helping to reduce VOCs that can be regulated in some areas or simply desirable for those trying to reduce chemical exposure to benefit employees’ health.
Additional additives or prep
There are additives we can use in some manufacturers’ primers or sealers to achieve different repair durability or characteristics. A common one is a flex agent for plastic parts such as bumpers to meet the demands for flexibility, expansion/contraction, and general abuse, such as stone impacts on the front of a car. Another use is for underpanel areas such as underhood or an inner bedside. These help give not only a factory-like matte inner panel color to replicate the e-coat appearance, for example, but also to help harden and seal the primer from the elements. Primers typically don’t have long-term durability without this additive if you’re not doing additional topcoating steps like color and clearcoat that is not always needed in such as in underhood or inner structures, which saves material and steps. An adhesion promoter may first need to be sprayed on plastics to help adhere the same primer (or sealer) you would use elsewhere.
Troubles? Tear down to the “foundation”?
With so many types of primers, many of the problems come from not understanding their uses in different areas and stages of the repair. We also may misuse the product by not following the product manufacturers’ TDS’ step-by-step requirements for the products' proper selection, mix ratio, and application. Develop shop SOPs to refresh everyone currently using the products and as employees shift to different roles in the shop or new employees are hired.
Simply put, let’s get back to basics on a few things regardless of if you’ve been spraying the products for years to decades or all the large number of products rolling through our shops by doing a few steps.
