Acrylic or epoxy primer — which to choose and why it matters more than you think

Acrylic and epoxy primers are two different products with different chemistry, different properties and different purposes. They are not substitutes for each other — they complement each other.

Acrylic primer is based on acrylic resin. Once applied, it forms a coating that fills unevenness well, sands easily and provides a good base for the top coat. It is versatile and simple to use.

Epoxy primer is based on epoxy resin which, once cured, forms a hard, dense and chemically resistant coating. Its main advantage is corrosion protection — significantly better than that of acrylic primers. It forms a barrier impermeable to moisture, which means that even if air reaches the coating, without moisture the metal will not oxidise.

Acrylic primers are the bread and butter of repair shops. They perform well over polyester fillers, levelled surfaces and old paint finishes prepared for refinishing.

Acrylic primers are available in white, grey and black. Choosing the right primer shade is not about aesthetics — it means real savings in base coat and better matching of the final colour.

For plastic parts — bumpers, mirrors, trim — an adhesion promoter is essential before the acrylic primer. Without it, even the best primer will not adhere properly to plastic and will come off the first time the part flexes.

Most acrylic primers can be used in three ways — depending on the amount of thinner added:

• Sealer version — minimum thinner, thicker consistency. Used to isolate body filler or an old finish before applying filler primer.
• Filler version — standard thinning. Fills fine scratches and unevenness; requires sanding before top coating.
• Wet-on-wet version — maximum thinning. After ten to twenty minutes of flash-off, paint can be applied without sanding — a time saving on smaller repairs.

There are situations in which acrylic primer simply is not enough. Epoxy is not a more expensive alternative — it is a product for different applications.

• Directly onto metal — in any repair where bare metal is exposed, epoxy as the first coat is the standard. Acrylic on bare metal is a recipe for corrosion that will appear after a few months or years.
• Under and over polyester filler — epoxy under the filler improves its adhesion to the metal. Epoxy over the filler isolates it from the subsequent coats and prevents discolouration from the peroxides contained in the filler hardener.
• On sanded old finishes — epoxy as an isolating coat prevents repair areas showing through the top coat over time. This is one of the most common reasons for complaints on repairs that look good straight out of the paint shop but start to show through after a year.
• On aluminium — aluminium requires special protection against electrochemical corrosion. Epoxy is the right choice here.

You can write that a primer is good. You can also prove it.

MAVAR primers undergo resistance testing in salt spray chambers in accordance with ISO 9227. The salt spray chamber simulates years of exposure to moisture and road salt within a few dozen hours — our primers come out of it without a trace of corrosion.

We also carry out tropical tests — they simulate the extreme high-temperature, high-humidity conditions typical of harsh climates. A coating that withstands tropical conditions will cope with a Polish winter and road salt without any problem.

These are not one-off tests carried out at product launch. Every batch of primers goes through the same procedures — so that you can be sure the product you open today performs exactly like the one we tested.

You can choose a cheaper primer of unknown origin. But you will not know what is inside or how it will behave in two years under your customer's paint. We know — and we can prove it with technical documentation.

To keep things simple — rules that cover 90% of repairs:

• Bare metal → always epoxy as the first coat. Never apply acrylic directly onto metal if you care about the durability of the repair.
• Polyester filler → epoxy under and over the filler. Isolating the filler is insurance against discolouration and adhesion problems.
• Levelling and filling → acrylic filler primer. On a well-isolated substrate, an acrylic filler primer will do the job best.
• Plastics → adhesion promoter before anything else. Without it, no primer will guarantee durability on plastic.

Use products from the same manufacturer throughout the system — primer, base coat and clear coat should come from a single product family. Mixing systems from different manufacturers is the most common cause of compatibility problems that cannot be predicted without testing.

And always check the product's technical data sheet before application. It is the definitive source of information on mixing ratios, flash-off times and compatibility. The technical data sheet is a contract between the manufacturer and the painter — if you follow it, the manufacturer is responsible for the result.