A lot of people doing their first at-home paint job hit their first stumbling block when it comes to buying primer. “Do I use epoxy, urethane, or polyester? Or all of them? In what order? And what is primer-surfacer?” These are all common questions, and we’ve put together this quick guide to help answer them.
What do primers do? Regardless of which variation we’re talking about, all primers serve the same basic function: to prepare the surface of a base material (metal, fiberglass, plastic) for maximum adhesion of paint, as well as body filler when needed, to ensure durability and a superior finish. Certain primers stick better to bare metal than others, making them an ideal first layer. Others are packed with higher amounts of solids content and are more sandable, making them suitable for fine-tuning any surface imperfections before finishing them with paint.
Epoxy Epoxy primers are the industry standard for covering bare metal and is therefore referred to as a ‘direct-to-metal’ or DTM primer. It is composed of two components – the primer base and a chemical activator – that must be mixed prior to use. Introducing the activator sets off a chemical reaction that cross-links the primer. This process not only cures the primer (as opposed to simply allowing it to dry) but also results in a superior surface bond.
Epoxy’s primary function is to bond to the base material and seal it for later work. Because it tends to lay down thinner and cure harder than other primers and fillers, sanding on this primer coat should be kept to a minimum, typically just a single pass with 80-grit to remove dirt or other imperfections and to reveal any low spots that appear after application. Afterwards, body filler can be used on areas that require it. Epoxy also works well over top of body fillers (good to know if you should uncover previous body repairs) and should be reapplied to seal any polyester filler before moving on to other coats of primer or paint.
Epoxy primers can be sprayed, rolled, or brushed on, but have a limited working time (a.k.a. – ‘pot life’) once they’re activated. This requires that the painter mix only what’s needed and can be applied during this window of time, and that any equipment be cleaned immediately after use. When working with epoxy, a proper ventilator should always be worn, as the application process will emit isocyanate compounds that can be harmful to the respiratory system.
Urethane primer-surfacer Following body filler and a good foundation of epoxy primer, the next layer you may want to apply will be high-build urethane primer-surfacer. This is an optional step, but is commonly taken to ensure the best finish. Urethane primer-surfacer offers a higher degree of solid material to fill in minor surface imperfections, such as sanding scratches from filler work. It can be built up in multiple light to medium coatings and then sanded back with successively finer (80 to 320-grit) papers to create flat surfaces and crisp body lines.
Like epoxy, urethane is also a two-component (commonly referenced as ‘2K’) activated material with a limited pot life and requiring immediate cleanup afterward. Likewise, proper ventilation is required for safety. Urethane primers are typically sprayed (either by a spray gun or aerosol can) but are now also available in a roll-on/brush-on formulation.
Polyester primer-surfacer Think of polyester primer-surfacer as a sprayable version of body filler. Just like traditional polyester filler, this is an activated product with a limited pot life. It’s intended to lay down heavy but sand out easily to produce great surface finishes.
Polyester primer-surfacer may used in place of urethane primer-surfacer when the project requires more surface shaping. Generally, the straighter and cleaner the sheet metal, the less likely you are to need polyester high-build primer. If you should use it, however, make sure to seal it with another coat of epoxy primer before moving on, just as with regular polyester filler.
Self-etching primer This final type of primer is a single-component (1K) non-activated DTM primer that would typically be used under enamels, lacquers, or other 1K paints. Self-etching primers contain an acid component to help it ‘bite’ into the metal for stronger adhesion. Typically sold in spray cans (but also available for roll-on/spray-on applications), this type of primer is best for small projects around the garage or the house, and it can be used with polyester fillers. While it offers improved corrosion resistance when applied before paint, it is not exclusively corrosion resistant on its own.