Let’s Talk About Thinners

To thin, or not to thin? That is the question. OK, that’s not a quote by Shakespeare, but if you’ve ever tried to paint on a hot day, it’s something that has gone through your head at least once! Thinners are key to a good finish.

What Are Thinners? 

Thinners are a substance which make paint ‘thinner’. They are made of a substance which is soluble to the product being used, which obviously varies according to the product. For example, adding water to acrylic-based paint or turps for oil-based finishes.

Why Add Thinners? 

We add thinners to paints and varnishes for a variety of reasons, depending on what we’re trying to accomplish. 

  1. If it’s a hot day, we can add thinners in order to extend the working life of the paint.
  2. To make the paint go further. 
  3. To change the consistency of paint or varnish, for example when using to make a glaze finish or fake marbling or wood graining, or to make special effects.  
Some thinners are used to extend drying time in order to allow finishes like Faux Marble

What Different Types of Thinners Are There?

There are different paint thinners for different types of paint. If you use the wrong thinners in the wrong type of paint, it will ruin the whole batch. Getting it right is essential!

Here are the different thinners to use with every paint type:

Water: The most common thinner – and the safest. Water is commonly used to thin water-based (acrylic) paints, as well as acrylic-enamels (water-based paints that act like enamels). 

Mineral Turpentine: Most commonly used for oil-based paints (enamels) and most polyurethane stain/varnishes. Also known as Turps,  White Spirit (UK) or Mineral Spirits (US/Canada), it is extracted as a by-product of the manufacturing process of petroleum. 

Penetrol: Made by Flood Company, Penetrol is a thinner for oil-based paints. Also commonly called a ‘paint extender’, it is made to not only make paint go further but to make the finish smoother and the coverage better. A benefit of using this in white paints vs turps is that it prevents ‘yellowing’. 

Floetrol: Known as an ‘Acrylic Paint conditioner’, Floetrol is used in water-based paints. Much like Penetrol, it extends paint, smooths edging, improves coverage and prevents increases drying time. 

When Not to Thin

It’s not advisable to use much (or none) water or turps as thinner for very light colours such as Dulux Vivid White, yellows or untinted paint. Because the tint is a large part of the coverage, these colours tend to have very little tint in them. Because of this, excess thinner will heavily reduce coverage, leading to problems such as picture-framing or the paint being see-through. 

How To Add Thinners to Paint

For most applications, just adding some of the relevant thinner is enough. Simply add the appropriate amount and stir.

After years of painting, a painter develops a kind of ‘sixth sense’ for how much thinner to add to each paint for varying situations. This takes a lot of experience and knowledge (and mistakes!). I’ll offer some advice on how to attempt this further down this article. 

How Much to Add

How much thinner to add depends on many factors, such as what you’re trying to accomplish, the temperature of the weather and whether it’s a windy day. As a general guide, however, we want to add between 5% and 20% of thinner to what we’re thinning. A safe bet is around 10%. In a normal 4L paint pot which is about 1/3 full, this would be a good splash of water or turps. 

For oil-based paints, there is a trick that pro painters use that I’m going to share with you: 

In a 4L paint pot that’s 1/3 full, add one capful of turps, and an equal amount of Penetrol.  Stir well. 

This mix offers the perfect amount for coverage and spreading the right amount, while preventing overspreading and extending drying time. Of course, it also depends on the type of brush you use. See our guide for everything you need to know about buying the right brush.

For airless spray, we use closer to 20% thinner. This is because the paint needs to move fast through the machine in order to give the right finish, and coverage is not a big problem because of the way that the paint is delivered to the surface. 

Mixing By Feel

As explained above, mixing in thinner without measuring requires a lot of experience and takes in a lot of different factors, and should be left to the professionals. If you want to try your hand at it, here are some tips:

TIP 1: Consider why you’re thinning. 

Is it because it’s a hot day and you want to prevent the paint from drying instantly as you spread it? Consider adding thinners at the upper limit, or add above the upper limit, but be prepared to use multiple coats. Is it to make the paint go further? Be aware that 10-20% is more than enough to stretch paint to it’s coverage maximum, and adding beyond that will result in needing to as several coats, unless it is a very solid, deep-base colour. 

TIP 2: Consider the colour.

Deep base colours tend to hold thinners well, whereas lighter colours such as white or yellow can deal with much less. As well, darker tint colours can change colour with overuse of thinner. Try to err on the side of using less than you think you may need. 

TIP 3: Be conservative

Most people think that when we add thinners to paint, we get a coverage area equal to the amount of thinners we add. This is not true: we get a lot more. Thinners work by changing the consistency of paint so that the spreading of it is more efficient. This means that a little bit of thinners really does go a long way. I’ve heard horror stories of inexperienced painters who use almost half the amount of paint in water. They need more than seven coats for the paint to cover! 

Bottom line: You need always need less than you think. 

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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