Confused by varnish

[The Q]

I don't think it's either sustainable or thrifty to throw away brushes. My late father, who lived through the austerity of WW2 and its aftermath always used to clean brushes after use, and I have continued the tradition. I have some brushes that are forty years old. The smaller ones are easier to clean, but tend to wear out more quickly.
I also often clean roller sleeves that have been used for water-based paints.
It is said in our family that we originally came to Ireland from Scotland in the 16th century.

Except that unless there has been an assessment done on the impact of producing the chemicals to clean the brushes and the waste that it produces verses the impact of making a new brush you don't actually know which is worse.

A bit like nappies, at one point they were pushing for the return to washing nappies, till someone proved the environmental impact of a disposable Nappy is much less than the boil wash needed to clean a reusable...

For me, water based paint brushes are always cleaned,
Brushes for enamel base paints get down graded, new one for top coats, a reused one that has been stored in turps/ white spirit for primers / undercoats that will be sanded before the next coat anyway..
Brushes for fibreglassing, forget it, use the cheapest and nastiest and throw it away..