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How Much to Thin Paint – A Guide

How much thinner to add to paint

When you are working on a painting project you often may wonder if you should add thinner and how much.  You may have had times where you didn’t add thinner and you couldn’t get the paint to lay out smooth or maybe you thinned the paint and then it resulted in runs all over your product.  This guide will help provide practical tips on determining how much to thin your paint to ensure you don’t run into problems with runs and still achieve a quality finish.


Step 1 to Determine How Much to Thin Your Paint – Check the Paints Data Sheets


Paint manufacturers have technical data sheets that provide recommendations on the maximum amount of thinner.  This can be a good starting point to help gauge the maximum amount of thinner that is recommended to add.  In general though you always want to start with as little thinner as possible and add a little at a time as over thinning will create a variety of problems like runs or even reduced paint performance.  So once you now the maximum suggested amount of thinner to consider adding then the next step is to determine the viscosity of your paint.


Step 2 – Determine the Starting Viscosity of Your Paint


The easiest way to thin paint and not overly thin it is to measure its original viscosity.  To measure a paints viscosity you can use a viscosity cup.  More on choosing a viscosity cup here. Typically a Zahn 2 is an easy cup to use as most air spray materials will flow well if they run 20 – 30 seconds in a Zahn 2.  Before thinning your paint you can get an initial viscosity.  If it is much above 35 seconds in a Zahn 2 you may need to thin it if your using a spray gun. With an airless paint sprayer, especially a higher pressure sprayer, you can often times not need thinner as much or at all.   The video below will help in showing how to use a viscosity cup and show you what different finish qualities look like so you can better know how much thinner yo may want to consider adding.

Step 3 – Do a test Spray

Once you have the base viscosity you should do a brief spray test to see your results.  If you find your paint finish chunky and have minimum fluid pressure as well as high air pressure, then you probably need to thin your material.


Step 4 – Add A small amount of thinner

If you have found that your paint is not spraying the way you hoped because it seems to not be atomizing well, then consider adding a minimal amount of thinner, if the technical sheet suggests 15% maximum consider starting with 1/3 of that amount.

Step 5 – Retest the material Viscosity or do a Test Spray

After adding the thinner you can recheck the material in a zahn 2 cup to see if its below 30 seconds which if it is it should then spray well.  You can continue testing the material after adding a slight bit of extra thinner until you achieve the result you want.

Ultimately what you want to do when thinning paint is try using the smallest amount of thinner that you can to get the result you need.  By adding thinner slowly and testing you will ensure that you get a good finish without having excessive thinner costs or excessively thinned paint that gets excessive runs or if you plan on using an air spray gun you can consider thinking about the 20 – 30 second time for a Zahn 2 cup as the ideal viscosity for air spray.  Once you have the amount of thinner you will use determined, record the amount you use to help build a repeatable paint process.

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