When painting walls, furniture, or other surfaces, it’s common for people to thin down the paint with water, paint thinner, or other solvents before applying it. The main reason for doing this is to make the paint flow and spread more easily when brushing or rolling it on. Many people also believe that diluting the paint helps achieve a smoother, less brush stroke-y finish. But is this really true? Does thinning paint actually create a smoother coat of paint compared to applying it straight from the can? Let’s take a closer look at the facts.
How does thinning paint work?
Paint is a mixture of pigments, which provide color, and a vehicle, which is the liquid that carries the pigment and allows it to flow. When paint is still wet, the pigment particles can settle and clump together. Thinning the paint reduces its viscosity, which helps break up those clumps. This makes the paint easier to apply smoothly without streaks or uneven coverage.
Most latex wall paints have a viscosity of 100-120 KU (Krebs Units). Thinning with 10% water, or about 1/4 cup per gallon, drops the viscosity to about 85 KU. Oil-based paints are normally around 70-90 KU, so thinning with paint thinner brings them down to around 60 KU. These thinner consistencies allow the paint to self-level better after application.
Common paint thinning ratios
| Paint Type | Thinning Liquid | Thinning Ratio |
|---|---|---|
| Latex wall paint | Water | 10% (1/4 cup water per gallon paint) |
| Oil-based paint | Mineral spirits | 10-15% (1/4-3/8 cup per gallon) |
So in short, adding thinning agent allows paint to spread out and level more evenly after brushing or rolling by improving its flow and reducing its viscosity. This helps minimize visible brush strokes and unevenness in the applied film.
Does thinned paint always give better results?
While thinning paint can certainly help it go on more smoothly, there are some downsides to be aware of. Over-thinning the paint reduces its hiding power and durability. Paint that is thinned too much may end up looking thin, uneven, or even transparent on the surface. High-quality paints are designed to provide maximum hide and film build at their out-of-the-can viscosity, so thinning strong brands like Benjamin Moore or Sherwin-Williams may not improve appearance.
Here are some pros and cons of thinning paint for a smoother finish:
Pros
- Improves flow and spreading ability
- Minimizes brush strokes
- Creates a more even, consistent finish
- Allows paint to self-level better
Cons
- Can reduce hide and make paint look thin if over-thinned
- Weakens the paint film, reducing washability and durability
- Requires careful mixing to thin evenly
- High quality paints are designed to be used as-is
So the bottom line is thinning paint can potentially improve the finish, but only if done properly and not overdone. It’s not always necessary or beneficial, especially with top-tier paint brands.
When should you thin paint?
Here are some situations when it makes sense to thin paint:
- Using inexpensive or economy grade paint – These low-cost paints are more likely to apply unevenly or leave visible brush strokes. Thinning 5-10% improves flow and self-leveling.
- Very warm conditions – Paint dries faster and is more likely to drag on hot days. Thinning makes application easier.
- Spraying paint – Paint sprayers require thinned paint, usually about 30% for latex and 20% for oil-based.
- Using dark colors – Deep tones like black or navy blue often cover unevenly. A little thinning improves hide.
- Applying glazes – Glazing liquids need to be thinned significantly (up to 20:1 ratio) to create a translucent effect.
However, there are also situations when paint should not be thinned for best results:
- Using quality premium paint – Expensive paints are engineered to be applied as-is.
- Painting over unprimed drywall – Thinning reduces critical hide and holdout.
- Cold, damp, or humid conditions – Thinned paint dries too slowly and can bleed or sag.
- Applying dark colors over light ones – Thinning reduces hide and may allow bleed-through.
- Seeking maximum washability – Thinning weakens the paint film and abrasion resistance.
So consider the specific paint product, project conditions, and desired results before deciding to thin.
How should you thin and apply paint for an optimal finish?
If you do choose to thin paint, follow these tips:
- Test on cardboard first – Thin incrementally and test coverage and appearance.
- Mix well – Stir or shake thinned paint thoroughly to blend.
- Apply multiple coats – Allow proper dry time between thinned coats.
- Use quality brushes/rollers – Purdy, Wooster, microfiber, and foam give smoother results.
- Maintain wet edge – Cut in edges first, then roll walls in 3×3 foot sections, keeping a wet edge.
- Roll lightly – Excessive pressure causes more stippling; use light, even strokes.
- Finish each coat consistently – Roll/brush in same direction and don’t overwork when drying.
- Sand lightly between coats – For furniture or trim; knocks down nibs and fuzz.
Proper application techniques are just as important as thinning for achieving a smooth finish.
How can you get a smooth finish without thinning paint?
If you don’t want to deal with the challenges of thinning paint properly, there are some alternatives to get a smooth brush stroke-free finish:
- Use quality premium paint – Like Benjamin Moore Aura or Sherwin-Williams Emerald Urethane Trim Enamel.
- Apply with a foam roller – Foam smoothes out paint thickness variations.
- Back brush – Follow rolling by lightly brushing over the surface.
- Spray paint – Airless sprayers provide the smoothest finish.
- Sand between coats – For trim and furniture projects.
- Use paint additive – Such as Floetrol for latex paint.
- Wet sand final coat – Gently sand the dried last coat to level dust nibs.
Using these techniques, you can often achieve professional-looking results without diluting the paint.
Conclusion
Thinning paint helps it flow and spread more easily, which allows it to self-level and minimizes brush strokes. However, over-thinning paint reduces hiding power and durability. The highest quality paints are engineered to be applied as-is. Lightly thinning lower grades of paint, as well as applying paint carefully using quality tools, keeping a wet edge, and back brushing, are the best ways to achieve a smooth professional finish. With higher quality paints and good application technique, thinning is often unnecessary. Test thinning ratios carefully and apply multiple coats for optimal results.