Should I Use Automotive Primer Before Painting? 5 Reasons Why Primer Paint Is A Must-Have

25 October 2022 Automotive

Do you need to use a primer before applying automotive paint? The answer is yes, in most cases. To understand when a primer is needed, let’s look at what it does.

Reason #1: Primer As A Rust Protector

Primer creates an additional impermeable layer that prevents salt, moisture and oxygen – the ingredients that will cause rust to develop – from reaching bare metal surfaces.

The best-performing primers for preventing rust are anti-corrosion primers, as they offer excellent adhesion to galvanised steel, stainless steel and mild steel and 400 hours of salt-spray resistance.

Reason #2: Primer As An Adhesion Promoter

Primer creates a strong bond with the underlying surface to ensure the colour coat adheres well and won’t peel or flake. Automotive primers are mixed in different formulations to create coatings that provide optimum adhesion to a wide range of surfaces.

For straightforward steel automotive panels, a conventional 1k epoxy primer spray is generally sufficient, providing the surface has been properly prepared and cleaned.

More difficult metal substrates such as alloy, stainless steel, Zintec and galvanised steel may require an acid etching primer.

Aluminium has quite different characteristics than steel and therefore requires an aluminium primer.

Painted steel and alloy wheels bear the brunt of all those stone chips and gritting salt that a car is subjected to, and should be treated to a specifically formulated wheel primer.

Metal welds also have their own primers, with separate formulations for copper weld primer and zinc weld primer.

Plastic bumpers, trim and panels also have different characteristics that affect adhesion. The good news is that plastic primers, while formulated for use on plastics are generally suitable for metal too, so can be used on all surfaces of the vehicle.

For the ultimate adhesion to plastic and non-ferrous metals, an adhesion promoter can be used.

Reason #3: Primer As A Filler

Minor scratches and sanding or grinding marks are often not bad enough to require filling putty or a urethane surfacer, and this is where acrylic primer fillers are invaluable.

Available as a 2k primer filler kit or as a 1k primer filler aerosol, they offer good build qualities and fast drying, while a super high build primer will allow an exceptionally deep layer of primer to be applied without running or sagging.

Reason #4: Primer As A Perfect Surface

Preparing a surface for painting often involves careful sanding of bare surfaces, old paint or automotive body filler to create a perfectly smooth surface that won’t reveal minor imperfections that are invisible until the high-gloss paint layer is applied.

Primers – and particularly filler primers – create a surface that can be easily sanded to a superfine finish without swirls or pinholes, creating the perfect surface in preparation for a professional colour coat.

And here’s a tip – use a dry guide coat to highlight imperfections in the primer as you sand it smooth, and you will eliminate the risk of discovering them when it’s too late!

Reason #5: Primer As A Sealer

If the paint is applied to a porous or semi-porous surface such as car body filler, the paint will be absorbed at a different rate than on surrounding surfaces, leading to dullness and inconsistency.

Primer acts as a barrier that prevents the paint from being absorbed, keeping it on the surface and ensuring a consistent finish across the entire area.

So there we have it – five reasons why primer paint is a must-have. But wait, you say – didn’t you mention there are cases where primer isn’t required?

When Primer Isn’t Required

Yes, that’s correct. Primer isn’t always needed and can even be excessive. So let’s look at the occasions when you don’t really need to use a primer.

If the repair is very small – and we mean really, really small – using primer might add layers that then need to be painstakingly sanded down to blend with the original surface. If the surface is sound, a light, localised sanding and a thorough cleaning might be sufficient preparation before you apply paint.

If the underlying metal or plastic isn’t exposed, this is another time when primer may not be necessary. Again, carefully give the surface light sand and a thorough clean before applying paint.

You could even use an adhesion promoter in place of primer, as this is a clear coat product that will make it easier to blend the new paint with the old.

But that’s it, basically. Most of the time you will be better off using a primer as it will give you a better-looking and more durable finish.

For more information on the ProXL range of car refinishing supplies visit and for help and advice contact us or call 01634 823900.