There has been quite a bit of discussion recently on both this and the other Canadian forum about finish thinners. Well, our son-in-law gave me a stack of magazines today to peruse (on loan) and one caught my eye with an article “Complete guide to thinners”.
It looks like I will save my money in the future and buy mineral spirits/paint thinner instead of turpentine, the two of which can be used interchangeably. It turns out that turps (more expensive and what I have been using) is variable in rosin content and the rosin content is not declared on the label. A high rosin content may leave a soft finish.
For the article see “Fine Woodworking”, October 2001. It has helped me.