AC Primer vs PU Primer

By Bradley Hext

AC Primer vs PU Primer

If your business spray-finishes timber substrates for internal joinery, you’ll be no stranger to Primer. Primers are formulated to fill the grain on solid & veneered timber and provide a flat base above MDF & similar surfaces for the Topcoat. They sand easier than topcoat itself, and are more cost-effective in comparison.

Here, we’ll explore the differences between two of the most common types of Internal Primers; AC (acid-catalysed) and PU (Polyurethane), to see which is the right choice for your company. 

What is an AC Primer?

Acid-Catalysed Primers use a similar base resin to Precat, but with the addition of an acid (the catalyst) to increase the speed of drying / curing. 

Most AC Primers uses an industry standard mixing ratio of 10:1, with variances from 5:1 to 20:1 depending on the exact product.

Characteristics of AC Primers
  • Long pot-life for use, often up to 24hrs
  • Touch dry in 30 minutes
  • Low price-point
  • Easy to sand once dry.
  • Typically requires additional solvent.
  • Easy to atomise via either low-pressure or air-assisted spray.
Where can AC primer be used?

AC primer is the perfect choice for all cost-effective finishing of a wide range of timber substrates including plywood, MDF, and composite products with exposed grain. 

When is AC primer NOT suitable?

AC primers are not suitable for use on external joinery such as windows/doors/orangeries. An excessively thick application in a single-coat can lead to cracking or lifting. Plus, AC formulations almost always contains formaldehyde, so adequate PPE must be used during application, and the solvent smell continues to exude from the painted surface for up to 4 weeks after application.

What is a PU Primer?

Polyurethane coatings comprise of two components: the base material, and an isocyanate catalyst, which must be added to the product before use. This creates a pot-life of around 1hrs to be used, before disposing of any un-used material before it cures. 

Most PU primers use an industry standard mixing ratio of around 2:1, with variances from 1:1 up to 4:1 depending on the exact product.

Characteristics of PU primers
  • Short pot-life, often around 1-3 hours.
  • The best performance in arduous environments.
  • Good vertical hold
  • Excellent sanding properties
  • High solid content
Where can PU primer be used?

Polyurethane Primer is perfect for furniture and joinery that’s subject to serious wear and tear. Suitable for use with MDF and solid timbers, its lower-solvent content means it’s also suitable for some plastics if required. PU also has good heat and moisture resistance which makes it an ideal choice for kitchens & bathrooms.

When is PU primer NOT suitable?

While PU products are formaldehyde free, the catalyst contains isocyanates which are harmful & can be ingested while atomised (during spraying). Therefore, it requires appropriate PPE to be worn, including mandatory breathing-air for the operator & anyone else in the spraying zone. Again, like an AC Primer they are not suitable for use on external joinery projects as the formulation is not microporous or flexible enough to follow dimension change of the substrate.

The key differences between PU and AC primer

Polyurethane primers tend have a higher solid-content than AC, which gives improved opacity & build per coat – Ultimately getting to the required level of finish in less time.

Modern PU formulations retain a very slight level of permanent elasticity making them less prone to cracking on joints than AC. However, the different chemistry combined with a thicker film requires a longer drying time.

The final word

Both PU and AC primers do a fantastic job to provide a strong base for paint but there are some subtle yet vital differences. Most tradespeople will have a preference for one over the other but, ultimately, the right primer for the job is dependent on the materials you’re working with and the methods you’ll use.

Check out our full range of primers here or get in touch with us to learn more 01452 525757, or fill out the form below.

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