Spray primers 

They are sold in cans or aerosol sprays and are used to carry out the first step of the painting process. These are the bodywork primers, which have various roles and uses :
Primers in cans are used to perfect a surface, but also to protect that surface from oxidation, or even to ensure excellent adhesion to that surface. No single can of primer can perfectly fulfil all three tasks at once.
The most common type of primer is the surfacer, which levels the bodywork before painting.

Primers known as surfacers are therefore thick and highly covering products, composed of a large proportion of filler (such as talc), which allows them to fill every defect in the surface and to be sanded effortlessly, leaving behind an entirely smooth and silky surface — whether it be sheet metal or fairings.
There are both single-component and two-component primers, available in cans and aerosol sprays (2K aerosols).

Primer aerosol sprays

Did you know that an aerosol spray marked 300 or 400 ml actually contains only about 100 to 150 ml of product? The remainder is simply the propellant gas. This makes aerosol sprays much less economical than paint supplied in cans.
However, many professional bodyshops are accustomed to using these spray primers, and they work perfectly well for them.

Aside from the reduced cost-efficiency and limited capacity of this packaging, its success can be explained by the time-saving factor: no spray gun to fill and clean, no mixing required... and in the end, a product that does not need to be applied with the same level of precision and care as Topcoat or 2K paint.

Is the two-component format of a primer aerosol better than the 1K spray?
Why formulate a primer with hardener when an aerosol can only be used once — since once the hardener button is activated, the product mixed in the aerosol has a shelf life of only a few hours?

The reason for 2K is that it is stronger: it allows for greater film build — probably double the thickness achievable with a single-component primer sold to the consumer market. The curing of the primer also allows it to harden and stabilise, creating a base that prevents the risk of reactions when applying solvent-based paints on top.

Priming with an aerosol spray

It is important to first assess the condition of the surface. There is no need to apply a thick coat to an older surface with only minor scratches.
On the other hand, it is necessary to apply as much build as required on a surface that has been repaired with filler and that presents numerous holes and cracks.

Primers impose far fewer constraints than paints, 2K paints or Topcoat (with their more limited coat tolerances). It is possible to apply more than two coats — for example, 3 to 5 coats. Beyond that, there is a risk of blistering in the dry primer film.

The different types of primers in cans

In some cases, it is necessary to use two different primers to prepare a surface — for example, a sandblasted motorcycle frame:
The first primer is designed to adhere to the metal and provide anti-corrosion protection, while the second primer provides build and levels the sandblasted frame. The metal primer, with a dry film thickness of 10 to 30 μm, cannot provide sufficient filling on its own.

Of course, it is possible to use a single primer directly on the metal in order to save time and cost, especially when maximum durability and performance are not required.
However, caution is needed: if the substrate is aluminium, carbon, chrome, plastic... anyone who skips the use of a dedicated primer for these materials risks experiencing peeling sooner or later.

Car body primer in a can
Coloring the chrome