Zinsser Cover Stain Primer Too Thick? Do This…

Oil-based primers are excellent for working on bare wood, metals, and glossy furniture. They tend to stick better than water-based primers for these surfaces. Oil-based primers also tend to be formulated to prevent tannin bleed, which causes yellow bleed-through in your top layer of paint. 

However, oil-based primers like Zinsser’s Cover Stain Primer can be thick and thus harder to apply. Fortunately, there is a workaround to this problem.

Is your Zinsser Cover Stain Primer too thick? In this post, we’ll cover how to thin this particular primer for easier application. 


If Zinsser Cover Stain Primer is too thick for your liking, you can mix up to 10% mineral spirits into the primer to thin it.

How to thin Zinsser Cover Stain Primer

Zinsser Cover Stain Primer is an oil-based primer that is best for wood with stubborn stains such as watermarks or tar from smoking.

This product is also great for blocking bleedthrough from tannin-rich wood. Tannins, which are acidic chemicals found in the sap of natural wood, can easily seep through primer if it’s not a stain-blocking solution such as Zinsser Cover Stain primer. 

According to this product’s technical data sheet (TDS), Zinsser Cover Stain is designed to be used without thinning. However, if you have a can that has been around for some time, the solution may have thickened over the years.

Normally, you can just thoroughly stir the primer before use to loosen it up. Alternatively, you can have the hardware store put it in a paint shaker to get the best consistency.

However, even after thoroughly mixing the primer, you may still feel it’s too thick for your liking. If your Zinsser Cover Stain Primer is too thick, use mineral spirits or paint thinners to thin it. Water can’t be used to thin it since water and oil-based primers don’t mix. 

To thin Zinsser Cover Stain:

  1. Transfer the amount of primer you need for your project to a separate paint container.
  2. Add a small increment of paint thinner or mineral spirits to the primer.
  3. Stir thoroughly 
  4. Check the consistency by brushing the primer on scrap wood or other material that matches your project.
  5. If the primer is still too thick, go back to step 2. 

Usually, the consistency will noticeably improve once you add about 5% of the can size worth of thinning solution.

For example, start by adding 6.4 fl. oz. of thinner or mineral spirit for a gallon of primer. Stir it in and check the consistency. If the primer is still too thick, add just a little more.

Be careful not to exceed 10% of the can size. Too much thinner can weaken the primer and affect the primer’s adhesion.

Note that oil-based primers tend to be generally thicker than other primers. Because Zinsser primers all behave differently and have their own unique benefits, consider assessing your project to see which Zinsser Primers is the best for your situation. For instance, Zinsser BIN and Zinsser 123 are two of Zinsser’s most popular products and may be more suitable solutions.

Can primer thicken up over time? 

Any primer can thicken over time due to the following factors:

Temperature: Whether it be water-based or oil-based, if the primer was left in cold storage or transported in cold weather, it may freeze, resulting in thick clumps. If this happens with a newly bought primer, it’s best to have it replaced. Otherwise, you may try to save it by using a sieve to remove the chunks, but this will not always guarantee good results.

Extreme hot temperatures may also cause thickening since solvents in primers will evaporate if the primer is left open.

Separation of components: Over an extended period of time, components in primers and paints sometimes separate, causing thick lumps. In some cases, it can be remedied by thoroughly stirring the medium to loosen up these clumps. Otherwise, it is better to get a new can of fresh primer.

Age: Like any paint medium, primers have a shelf life. Primers that have passed their expiration date will likely begin to break down. Generally, most unopened primers are good to use within 2-3 years from their manufacturing date. On the other hand, opened primers (despite proper storage) may only be good to use within 6 months to a year.

Contamination: Opened primers are more prone to contamination from dust and dirt. And if they’re contaminated, they’re more likely to form lumps and thicken even when properly stored.

How to prevent primer from thickening

As mentioned above, temperature and age are the major factors that lead a primer to thicken. To prevent this incident from happening, consider these tips:

  • Pour however much primer you need at once in a separate container to avoid contaminating the entire container with dust or dirt. Contamination can lead the primer to thicken over time. 
  • Always tightly close the container properly after use.
  • If only a small amount of primer is left, transferring the medium into a small paint container or mason jar is best. This will minimize air exposure.
  • Unopened and opened primers should be kept at room temperature.
  • Use your primer within 2-3 years after you’ve purchased it and use it up within 6 months after you’ve opened it

How to apply Zinsser Cover Stain Primer 

Zinsser Cover Stain Primer is straightforward to use.

  • Gather your supplies: primer, a stirrer, and a brush or foam roller 
  • Wear protective gear to prevent eye, nose, and skin irritation.
  • Prepare your priming area by covering the floors with old newspapers, drop cloth, or a tarp to prevent damaging floors. Also, open the windows to improve air circulation.
  • Clean the surface you intend to prime to remove dust, dirt, and other contaminants. 
  • Scuff sand with medium to fine grit sandpaper to smooth over imperfections, and to create a more textured surface for the primer to grip to. Clean off the surface once more to get rid of sanding dust. 
  • Open the can and pour the primer into a separate container. Only transfer what you need for the project.
  • Stir the primer and add in mineral spirit if the primer is too thick. 
  • Use a brush or roller to apply the primer on the surface. Apply in thin layers, making sure to go along the wood grain if possible.
  • Let the first coat dry for about two hours.
  • Once satisfied with the primed layer, you can apply your paint of choice.

If using the aerosol form of Zinsser Cover Stain, follow the same steps to prep the furniture. When it comes to spraying, always spray in thin layers and in short, dusting strokes. Also, occasionally shake the can for a minute to prevent blockage and ensure an even application. 




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