Paint Primer Basics for Classic Cars

A classic car paint job is only as good as its foundation. Do not use a cheap primer under expensive topcoats. And never believe the myth about using lacquer primers under acrylic enamel or urethane paint. Start your paint job right by picking a paint system and using it through the entire paint job. Too many things can go wrong with paint, so give yourself an advantage by using products designed to work together so following along in our paint primer basics.

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

In the automotive world, the term primer generally refers to a paint-like substance that is usually applied to freshly sanded metal before the topcoat. Much like paint, different types of automotive paint primer can be applied with a spray gun or a brush and can dry thoroughly between coats. While it might seem like adding an unnecessary step, using paint primer when refinishing an auto body is important for several 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 paint primer before completion of the final paint job.

Why Use Paint Primer?

First and foremost, primer helps the paint adhere to bare metal or fiberglass. Without primer as a buffer, the shiny metal surface, whether old or new, will not bond with the paint well. This leads to peeling, flaking, and eventually rust, which turns a drive-able car to junk in no time. Automotive primer acts as a bonding agent to help the paint adhere more strongly to the body of the vehicle.

Equally as important, paint primer helps prevent rust and moisture damage to your vehicle by adding a couple of extra protective layers. This is generally a sealer type of primer, 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 Automotive Primer

If you are performing any type of bodywork, repairs, or paint updates, you will need to use primer at some point. This is especially true if you are refinishing your car’s paint job at home and will be sanding down to bare metal or removing rust. Most 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 have mixed your primer, before you proceed with priming any part of your vehicle, you will want to make sure you have 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 have selected a primer that is known for good build, like polyester primer.
Sand down any blemishes or inconsistencies before priming, and especially after filling or using putty.
Once you have completed the sandingand other prep, you will 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 are just performing small touch-ups, brushing the primer on by hand will work, and you will want to use smooth, even strokes to avoid visible lines in the end paint job. If you are going to be painting your entire vehicle or repainting larger portions of the car, using a spray gunwill 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 are finished to prevent build-up on your equipment.
Once you have 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 are not certain the primer coat is completely dry and allow extra time if needed.
The number of primers coats you will need will also vary. For large areas and whole-body work, two coats are 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 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 is formulated specifically to prevent corrosion, so epoxy primer will not 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 are using to perform repairs and is applied over a secondary base primer as it does not provide the best corrosion resistance.
Polyester Primer– Polyester auto primer has what is 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 does not really have any filler capabilities but is perfect for when you are painting a vehicle that is already in decent condition, or you need to seal off a good amount of filler or bodywork.
Acid Etch Primer– Another good, basic auto primer. Acid etch primer is much like urethane surfacer in that it is 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 an 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 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.

For your classic car, the foundation is as important as the finish coats so no matter whether it is a DIY project, or your classic car is off to the paint shop the paint primer is a key part of that top notch paint job.