All About Automotive Primer

Primer. It's an important part of most painting processes, and this is no exception in the world of automotive paint. Whether you're painting your own vehicle or just performing touch-ups, most will recommend sanding and then applying a primer before continuing with the final topcoat.

So - What IS Primer?

In the automotive world, the term primer generally refers to a paint-like substance that's usually applied to freshly sanded metal before the topcoat. Much like paint, different types of automotive primer can be applied with a spray gun or a brush, and is allowed to dry thoroughly between coats. While it might seem like adding an unnecessary step, using car primer paint when refinishing an auto body is important for a number of reasons. Some will have good filling capabilities, some will provide sealing against the elements and others will work best when used in conjunction with a second type of primer before completion of the final paint job.

Why Use Primer?

First and foremost, automotive primer helps the paint adhere to bare metal. Without primer as a buffer, the shiny metal surface, whether old or new, won't bond with the paint well. This leads to peeling, flaking, and eventually rust, which turns a driveable car to junk in no time. Car paint primer acts as a bonding agent to help the paint adhere more strongly to the body of the vehicle. 

Equally as important, auto paint primer helps prevent rust and moisture damage to your vehicle by adding a couple of extra protective layers. This is generally an automotive primer sealer, like urethane or epoxy-based.

It also can serve as a filler for sanding/grinding marks and small scratches on your car body, negating the need for putty or a more prolonged repair job, like a urethane surfacer.

How to Use Auto Primer

If you're performing any type of bodywork, repairs or paint updates, you'll need to use primer at some point. This is especially true if you're refinishing your car's paint job at home and will be sanding down to bare metal or removing rust. Most auto body primer types come available as "two-part", which means a primer base and an activator need to be mixed before use. Simply follow the included instructions, and alter as necessary. Others, like urethane primer, can require up to 4-parts mix ratio, but easy-to-follow instructions are provided. Still others come complete in aerosol form for quick and easy application.

Once you've mixed your car primer paint, before you proceed with priming any part of your vehicle, you'll want to make sure you've accomplished a few preparatory tasks first:

  • Remove any rust completely, either by sanding by hand or grinding.

  • Fill any large dents, dings, scratches, or pock marks on your vehicle's body, unless you've selected a primer that's known for good build, like polyester primer.

  • Sand down any blemishes or inconsistencies before priming, and especially after filling or using putty.

  • Once you've completed the sanding and other prep, you'll want to make sure the vehicle's surface is as clean and free of particles as possible to allow for better contact. Give your vehicle a quick wash and spot clean any areas you may have sanded or ground down with a wet rag. As always, allow the vehicle surface to dry completely before applying any type of paint or primer.

  • How you apply the primer will depend on the scope and size of your project. If you're just performing small touch-ups, brushing the primer on by hand will definitely work, and you'll want to use smooth, even strokes to avoid visible lines in the end paint job. If you're going to be painting your entire vehicle or repainting larger portions of the car, using a spray gun will be your best option. Always start with a clean spray gun, and keep a bucket of thinner on hand in which to soak spray gun parts as soon as you're finished to prevent build-up on your equipment.

  • Once you've applied the auto primer, curing time will vary between types, so make sure you read labels and instructions. Smears and poor curing jobs lead to poor paint jobs, so err on the side of caution if you're not certain the primer coat is completely dry and allow extra time if needed.

  • The number of primer coats you'll need will also vary. For large areas and whole-body work, two coats is the standard. This allows maximum coverage and rust prevention, and provides the best base for paint adhesion. For smaller touch-ups, use your best judgement. Only one good coat might be necessary.

Different Kinds of Automotive Primer

The kind of car paint primer you end up using will vary with your project needs. You can ensure a lasting, high-quality auto paint job and extra rust protection with a good primer base. Different types of automotive primer will also withstand sanding differently, and depending on your project, you may want to consider this.

  • Epoxy Primer - Epoxy primer is considered a good standard base when it comes to ensuring the auto paint bonds to the metal and promotes a quality paint finish. It's formulated specifically to prevent corrosion, so epoxy primer for cars won't sand as well as other types, like urethane primer.

  • Urethane Primer Surfacer - This type of two-part primer surfacer is often used in conjunction with any putty or fillers you're using to perform repairs and is applied over a secondary base primer as it doesn't provide the best corrosion resistance.

  • Polyester Primer - Polyester primer has what's known in the auto world as excellent "build" - it fills small scratches and dings much like a putty or filler would and has the highest filling capability of any sprayable primer. This makes it perfect for filling bodywork blemishes AND achieving a good paint bonding surface at the same time. However, it tends to be a bit more brittle and prone to cracking than urethane or epoxy once dry, so this makes a great primer for small repair and filling jobs but may not be the best choice for an entire car.

  • Urethane Sealer - This type of primer is best used merely as a strong adhesive layer for the paint to bond to. Urethane sealer doesn't really have any filler capabilities, but is perfect for when you're painting a vehicle that's already in decent condition or you need to seal off a good amount of filler or bodywork.

  • Acid Etch Primer - Another good, basic car paint primer. Acid etch primer is much like urethane surfacer in that it's forte is not so much corrosion prevention as providing a strong bonding surface for paint. If extra rust prevention is the goal, use acid etch primer in conjunction with a sealer or rust preventative. This type of primer dries much more quickly than the others, so is used in a lot of body repairs in auto shops to speed up repair time. It also eliminates the need for any metal conditioner, so this is best when applied directly to bare metal and then covered with a secondary primer, like epoxy or urethane.

  • Enamel Primers/Sealers - Enamel primers are a extremely economical and, much like epoxy, provide a good base for auto paint to adhere to. They have a fair to good level of corrosion resistance.

  • Lacquer Primers/Sealers - Lacquer primer dries quickly and sands quite well, but can lead to cracking and bubbling in the long term, so these auto paint primer types are best used beneath a corrosion prevention and paint layer for small bodywork jobs.

  • Moisture Cure Urethane Primer - This auto primer is excellent for both paint adhesion and rust protection on difficult repair jobs where complete rust removal is unobtainable, making it an excellent all-around primer choice for completing two jobs at once. It also cures quickly when exposed to the moisture in the atmosphere, so curing times are cut roughly in half.

When To Use Automotive Primer & When You Might Not Need To

Whenever you're dealing with bare metal, whether old or new, you'll need to use primer before covering the area with any automotive paint. If you're making small body repairs and need to sand down or grind out a blemish, it's important to protect that area and make sure the paint will bond with the surface in order to further damage from rust or peeling.

The only time you won't need to use primer is if you're not uncovering any bare metal. If you're just lightly buffing away the top layer of paint and haven't uncovered your vehicle's steel panels, then it's ok to forego the primer. This goes for any plastic parts as well. If you're not removing paint down to the bare surface, then the primer is unnecessary.

So...What Primer Should I Use?

If you're needing a best all-around primer, something that provides a little corrosion protection and allows maximum paint adherence these will be some of your best bets:

  • For large paint jobs - in instances where you need to repaint/refinish a large surface area of your vehicle, epoxy primer will usually be your best option. It's a two-component primer, so it's simple to mix, and provides the optimum blend of paint adhesion and corrosion resistance and protection. Epoxy auto primer can be applied on top of everything from fillers and fiberglass to prepared steel or factory finishes. Dry time is quick as well, making this a great all-around auto primer for home mechanics and auto body shops alike.

  • For small touch-ups - polyester primer surfacer is perfect for those little repair jobs that require some light filling or putty, as it has great "build" - meaning it's on the thicker side and has the ability to fill in minor nicks and scratches and sands down well, negating the need for an extra putty or filler step. Polyester auto primer is perfect for completing small auto body repair jobs and is excellent for spot repairs.

  • For the best rust protection - in situations where rust was present and sanded out, or even where complete rust removal is not an option, moisture cure urethane primer will provide the best protection from further rust damage. Urethane auto primer sands easily, dries quickly, and holds color well.

Deciding which type of automotive primer to use doesn't have to be overwhelming or confusing. Begin by determining which needs are most important when it comes to repairing or repainting your vehicle - will you need extra rust prevention? Is strong paint adhesion a priority? If you still have any questions, Auto Body Toolmart is happy to help! Contact us to help you troubleshoot your primer choices, or browse our entire selection of automotive primers and paint supplies.

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