Thinning polyurethane is a common practice for woodworkers and painters.

It is necessary to thin polyurethane to coat the surface to be protected properly. If it is not thinned, the polyurethane will be too thick and will not dry properly, leading to peeling or cracking.

In this blog post, we will teach you how to thin polyurethane safely and effectively. We’ll talk about the best polyurethane thinner and the correct ratio.

Stay safe and have fun with your projects!

How to Thin Polyurethane

When thinning polyurethane, first stir and transfer the poly into a glass jar. Next, add in the suitable thinning medium. Finally, stir again to mix the products properly. Use water or denatured alcohol to thin water-based polyurethane. And use mineral spirits, paint thinner, or turpentine to thin oil-based poly.

How Much Can You Thin Polyurethane?

Polyurethane thinning ratios depend on the type of polyurethane, the intended mode of application, and the size of the surface.

For polyurethane that will be applied using a spray gun or paint sprayer, thin it at 10% for water-based poly and up to 50% for oil-based polyurethane. In any case, thin it slowly while checking the consistency and adjusting the ratio as desired.

However, if you’re thinning poly to apply with a paintbrush, mix the product using one part thinner to three parts polyurethane.

Always ensure polyurethane is more than the particular thinner, except when you have to put half mineral spirits and half polyurethane.

While diluting polyurethane, pour the thinning agent into polyurethane and not the other way round.

Related Post: Can You Dilute Wood Stain to Make It Lighter?

How to Thin Water-Based Polyurethane with Water 

First, open the polyurethane can and mix the product using a stirring stick. Next, pour a portion of it into a glass jar and then add water into the poly. Stir to mix the products properly.

Use 10% water for the total amount of polyurethane used. That’s the snippet answer for those who are in a rush. But for a detailed explanation of the procedure and materials, you’ll need.

Supplies for thinning water-based polyurethane

  • Water-based polyurethane
  • Water or denatured alcohol
  • Clean stirring stick
  • Glass jar

The steps

Step 1: Ensure the glass jar or plastic container is clean

Wash the glass jar if you need to and let it dry or wipe it with a lint-free cloth.

Step 2: Open the polyurethane can and stir it

Open the water polyurethane can use a paint can opener. Water-based polyurethane looks milky white. But if that’s a bit translucent, some pigments may have settled at the bottom.

Dip a clean stirring stick and mix the thick product simulating a figure 8. Mix gently to avoid creating air bubbles. Stop when the product is well mixed.

Step 3: Measure and pour the water-based polyurethane into the jar

Ensure your measurements are precise, so you know the right amount of thinner needed. In which case, use a measuring cup for accuracy. If you’ve done it many times before, you can eye-ball the amount of poly you’ll need.

Step 4: Measure the thinning medium and add it to the jar

The amount of polyurethane you put in the jar dictates the amount of water you need. For example, say you went with 4 cups of polyurethane; 10% of that will be 0.4 cups of water (95 milliliters).

Measure it using the measuring cup and pour it slowly into the jar. Adjust the ratio if you’re covering a large surface.

Step 5: Stir the poly and water mixture

Stir the mixture to dilute polyurethane. Keep stirring for about 3 minutes for the contents to mix well. Once the mixture is thinner than its initial state, the process is complete.

Step 6: Test the thinned polyurethane on wood

Apply a coat of the thinned poly using a brush, lint-free cloth, or spray gun. It should flow and not run. If you like the results, apply them to the rest of the wood.

Trans-fill any left-over material into a small bottle and seal it tight –it lasts longer.

Read also: How to apply polyurethane to floors.

How to Thin Oil-Based Polyurethane

First, open the polyurethane can and stir it properly. Next, measure and pour polyurethane into the jar. Next, pour some mineral spirits into the jar and gently stir the mixture. You can use a 1:1 or 2:1 polyurethane for mineral spirits.

That’s the brief procedure of thinning oil-based polyurethane if you’re skimming through. But if you like the detailed explanation, see below.

What you’ll need

The steps 

Step 1: Ensure the glass jar or plastic container is clean

Wash the glass jar if you need to and let it dry or wipe it with a lint-free cloth.

Step 2: Open the polyurethane can and stir it

Open the oil-based polyurethane container using a screwdriver or a paint can opener. Next, stir to mix the product using a stir stick or a ruler. Mix the product simulating an “8” kind of motion. 

Keep stirring for about 3 to 5 minutes. Do not shake the product to avoid creating polyurethane air bubbles that will ruin your project.

Step 3: Measure and pour polyurethane into the jar

Once you’re done stirring your polyurethane, pour it into the measuring cup before transferring it into the glass jar, plastic bottle, or sealable container. Add more polyurethane, depending on the size of your project. 

Step 4: Measure the thinning medium and add it to the jar

Next, measure a cup of mineral spirits or thinner and add into the oil-based poly container. This is basically a 1: 1 ratio, especially if you’re making wipe-on polyurethane.

But if you want a slightly thinned poly for brushing, make it 2:1 polyurethane to mineral spirits.

Step 5: Stir the poly and mineral spirits mixture

Once again, stir the poly and thinner using a stir stick. Mix the two products slowly to avoid creating bubbles. Stop stirring when you can tell the oil-based polyurethane is lighter and has a uniform flow.

Step 6: Test the thinned polyurethane on wood

Spread the poly on wood using a brush or cloth. Then judge if the consistency is right or requires more of either thinner or polyurethane. Apply it all if you’re satisfied with the consistency.

But if you have prepared it for future use, replace the lid and tightly seal the glass jar to keep polyurethane from going bad while in storage. 

Tip: Label the container with the type of polyurethane and the thinning medium used.

How to Thin Water-Based Polyurethane for Spraying 

Water-based polyurethane is considerably thin enough to spray without diluting. But if you want to, use 5-10% water of the total polyurethane volume.

First, stir polyurethane to mix up the product. Next, measure and pour water into the polyurethane and keep stirring. Finally, transfer the thinned poly into the spray bottle using a filter funnel to remove any contaminants when it’s all mixed up.

Alternatively, you can buy ready-to-use aerosol spray cans for spraying water-based polyurethane

Related: Can you use water-based polyurethane over oil-based polyurethane?

Polyurethane Thinners: What Do You Thin Polyurethane with?

How to Thin Polyurethane Image illustration

You can use several thinning mediums to thin polyurethane or wiping varnish. For example, water, denatured alcohol, mineral spirits, turpentine, and paint thinner. 

Here are more details.

1. Water

You can only use water for thinning water-based polyurethane and never the oil-based counterpart. The primary reason is that water-based poly uses water as a base and carrier for paint particles.

And so water won’t affect the paint’s texture or its viscosity. Also, note that this is the safest thinning agent, just like the water-based polyurethane itself.

Interesting read: Can you apply water-based polyurethane over oil stain?

2. Denatured Alcohol 

You can use this solvent to thin polyurethane, even though it’s most suitable for diluting water-based polyurethane. Like water, alcohol doesn’t mix well with oil and would only ruin the product.

Before using denatured alcohol, ensure you’re in a well-ventilated space and wearing gloves. This solvent has strong fumes and may irritate bare skin. 

Thin polyurethane using the exact amount of denatured alcohol you would with water.

3. Mineral Spirits

Use this mild solvent to thin for oil-based polyurethane and other oil-based varnishes. Mineral spirits is a petroleum distillates and contain oily resins obtained from trees.

The quantity of mineral spirits you will use depends on how thin you want your product. Although there’s a low-odor variety of mineral spirits available, always use it in a well-ventilated area while wearing gloves.

4. Turpentine

Turpentine is another suitable thinning agent for oil-based polyurethane finish. However, it’s more common for thinning oil-based polyurethane paint.

Use the same amount as mineral spirits to thin oil-based polyurethane. Turpentine is a heavily scented solvent with a toxic smell that doesn’t dissipate easily.

Therefore, you should only use it outdoors and wear gloves, long sleeves, and a respirator mask. Overexposure to the fumes can trigger headaches and skin irritation.

5. Paint Thinner

Use paint thinner to thin polyurethane. There’s a water base and an oil base variety. You should use a paint thinner matching the base of the polyurethane formulation.

Simply put: use water-based paint thinner for water-based poly and oil-based paint thinner for oil-based poly. Using the wrong paint thinner is a recipe for one of the bad polyurethane jobs.

Read also: How much time should I wait between applying coats of polyurethane on hardwood floors?

What Happens If You Over-Thin Polyurethane?

One of the risk factors associated with thinning polyurethane is over-thinning it; there are several consequences. First, over-thinning polyurethane makes the paint or finish too light to apply.

This is especially true for tintable polyurethane. You’ll notice that the product’s original color is far from what it was, and it’ll be pointless to apply it.

Another consequence of over-thinning is having the product run and drip all over during application. This is a nightmare, especially when working on vertical surfaces like doors and walls.

Also, overly thin polyurethane coats won’t adhere to the surface. As a result, it’s likely to flake and peel off within 6months of application. What a waste, right?

Another potential outcome of over-thinning is that polyurethane won’t be as water-resistant as it should -predisposing the surface to damage. And if that’s not enough, expect polyurethane coating to look dull and blurry.

Strictly follow the recommended thinning ratios. But if you think you’ve over-thinned the polyurethane, you can salvage it by adding more polyurethane.

Read also: Can you paint over polyurethane?


How to thin polycrylic?

Stir, measure, and pour polycrylic into a clean container. Next, measure and pour water or water-based thinning medium into the polycrylic. Mix gently but thoroughly. Once you get your desired consistency, test it on scrap wood before proceeding with your project.

Can you thin water-based polyurethane? 

You can thin water-based polyurethanes using either water or denatured alcohol. You will need to thin the overall water based polyurethane volume with about 5 to 10% water or denatured alcohol. Add the thinner into the poly gradually while stirring to avoid over-thinning.

Can you thin polyurethane with paint thinner?

Yes, you can thin polyurethane with paint thinner. Get oil or water-based paint thinners for the respective polyurethane carrier. Ensure you use the correct measurements and stir gently to prevent bubbling. Another alternative you can use is mineral spirits. The difference between mineral spirits and paint thinner is very subtle. The main distinction lies in the refinement process, with mineral spirits typically being more refined and having fewer impurities than traditional paint thinner.

How to thin water-based polyurethane for wiping?

Transform water-based poly into wipe-on by adding 10-15% water into your polyurethane. This quantity will make polyurethane thin enough to apply with a rag. However, it is always recommended to apply water-based polyurethane using a paint or foam brush and a spray gun.

Can I thin water-based polyurethane with Floetrol?

Yes, you can thin water-based polyurethane with Floetrol. This product is an additive primarily designed for latex paints, but it works with water-based finishes. Floetrol improves the paint’s consistency, flow, self-leveling features, and weather resistance.

How to thin Minwax polyurethane?

Begin by stirring Minwax polyurethane to mix up the contents properly. Next, add the recommended amount of the thinning medium into the polyurethane container. Gently stir the mixture until you get the right consistency. Other solvents you can use include; mineral spirits, turpentine, denatured alcohol, paint thinners, water, and lacquer thinner for oil-based varnish.

Find out: Can you put polyurethane over lacquer?

Wrapping Up on How to Thin Polyurethane

In conclusion, thinning polyurethane comes down to two critical things: use the correct thinning medium and the correct thinning ratio. Any slight alterations with the thinner will result in over-thinning- nobody likes that! 

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