How To Undercoat Like a Pro

There’s one thing that means the difference between a professional finish and one that looks unprofessional – the undercoat. Here’s what you need to know about how to undercoat in a way that leaves an impressive finish.

Lack of undercoat results in things like peeling and cracking
Lack of undercoat results in peeling and cracking

Why is undercoating so important anyway?

An undercoat is basically any coat of paint underneath the final coat. A primer coat can also be an undercoat. 

Typically, we use a sealer/primer for all coats of undercoat. It usually has little to no gloss level. This ensures solid coverage, which is important in a solid-looking finish. It also means that it sands easily and smoothly, which means a better-looking (and feeling) finish. 

Why can’t I use a topcoat for all the coats?

Typically, a topcoat is a high-gloss product. High gloss paints look fantastic, but the more gloss is in a paint, the harder it is to sand or cover. High-gloss provides almost no coverage, just colour and shine. We use undercoats of sealer/primer in order to provide the coverage and sandability. Sanding paint from low-sheen upwards to high-gloss takes a lot of work, and leaves marks and scratches which will likely show through. It’s also a lot harder for fillers and putty to adhere between coats – and they’ll show through anyway. 

Let’s go through the process.

Use a similar colour to the topcoat

First thing’s first – make sure that the colour of your undercoat is at least similar to the colour of your topcoat. If it’s too different, it will show through. Again, a topcoat is rarely designed fo coverage – there’s nothing worse than undercoating a door in green, only to remember that the topcoat is white or yellow. The closer the undercoat colour to the topcoat colour, the more likely it will be to cover – saving you money and time on extra coats.

Mixing layers of colour is not usually a good idea
Mixing layers of colour is not usually a good idea

The more coats the better

This is simple – the more coats of undercoat, the better the topcoat finish will look. One of the roles of undercoat is to ensure that the finish looks solid and brilliant. 

Because of a sealer/undercoat’s thick viscosity, it flows out evenly, filling out minor imperfections, and raising the surface fibres which allow for more to be sanded off between coats. This allows for a smoother and more visually appealing finish.

The quality of finish depends heavily on what happens BETWEEN coats.

This may be the most important thing to know: It’s what happens between coats that creates the quality of finish.

How well it’s sanded, dusted, and filled all impact the end result. 

After the first coat has completely dried, sand as well as possible. You want to spend some time doing this – the better sanded your surface is, the better the end result. For all the details, see our post on sanding to perfection. 

Don’t be scared to thin

Thinning is especially important when undercoating bare metal
Thinning is especially important when undercoating bare metal

Primer/Sealer Undercoat is something that usually takes well to thinning. The plain product is usually quite thick, and it’s safe to add up to around 20% thinner in order to make application easier and absorption faster. Be mindful of using the right thinners for the right product, and don’t forget that more thinner means more over-spray and splashes. 

If undercoating is done, 70% of the work is done right. That’s less time needing to worry about a perfect finishing technique.

Even better, it’s fun! 

Happy painting.

PaintSpace Admin

Christos is a fully-qualified Painter and Decorator with a Certificate III in Building and Construction. He has been working in the industry since 2004. Having studied Health Science at Torrens University in Melbourne, Christos is passionate about the cutting edge between health and painting.

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