I’d like to get some opinions or viewpoints on thinning finishes, ratio and percentages.

 Ratio is very straight forward, 1 part varnish to 1 part thinner, 2 parts varnish to 1 part thinner, etc. 

 Percentages though is where I go back and forth. Is it a percentage of thinner based on the amount of unthinned varnish or percentage of thinner based on the total amount of thinned varnish?

Say you’re looking to thin 50%. If you have 8 oz of varnish do you add 4 oz of thinner so you have half as much thinner as varnish (2:1), or do you add 8 oz of thinner so half of the total mix is thinner (1:1)?

1 hour ago, Zack said:

Ratio is very straight forward, 1 part varnish to 1 part thinner, 2 parts varnish to 1 part thinner, etc

In the example, 2 parts varnish to 1 part thinner, the percentage would be to the total. 

So varnish to total is 2:3 or 67%, thinner to total is 33%.  So wouldn't 50% be 1:1?

From ChatGPT-

 

"If you’re looking to thin the varnish by 50%, you would add 8 oz of thinner to the 8 oz of varnish, resulting in a 1:1 ratio (equal parts varnish and thinner). This means half of the total mixture would be thinner, achieving a 50% dilution."

And from Microsoft Co-Pilot-

"When mixing varnish with thinner, the percentage can be calculated in two different ways depending on the context:

Percentage of Thinner Based on Unthinned Varnish: This approach calculates the thinner as a percentage of just the unthinned varnish. For example, if you have 100 ml of varnish and add 50 ml of thinner, the thinner would be 50% of the varnish.

Percentage of Thinner Based on Total Mixture: This method determines the thinner as a percentage of the total mixture (varnish + thinner). Using the same example (100 ml varnish + 50 ml thinner), the total mixture would be 150 ml. The thinner's percentage in this case would be 50150×100=33.3%\frac{50}{150} \times 100 = 33.3\%.

It’s always good to clarify which method is preferred, as both are valid but can lead to different interpretations!"

Thanks, that’s what I was looking for. Percentage thinned is based on the total volume of the product including the thinner.

17 minutes ago, lew said:

And from Microsoft Co-Pilot-

"When mixing varnish with thinner, the percentage can be calculated in two different ways depending on the context:

Percentage of Thinner Based on Unthinned Varnish: This approach calculates the thinner as a percentage of just the unthinned varnish. For example, if you have 100 ml of varnish and add 50 ml of thinner, the thinner would be 50% of the varnish.

Percentage of Thinner Based on Total Mixture: This method determines the thinner as a percentage of the total mixture (varnish + thinner). Using the same example (100 ml varnish + 50 ml thinner), the total mixture would be 150 ml. The thinner's percentage in this case would be 50150×100=33.3%\frac{50}{150} \times 100 = 33.3\%.

It’s always good to clarify which method is preferred, as both are valid but can lead to different interpretations!"

Ok, looks like my last reply was in the works at the same time you posted this.

That’s the exact query I had starting this post. It would be nice if there was a standard. 
I’ve always done it as a percent reduction of the unthinned component. 50% thinned is 8oz varnish to 4oz thinner.

Edited March 15, 20251 yr by Zack

I have always taken percent as to the whole. So thin to 75 % would be the same as 3:1 . Usually the stated percentage is the percent of the total that is the primary finish.