"You Can Add All the Thinner You Want!"

This is absolutely true, as long as the thinner is compatible with the specific type of finish (cred- I was a 100% solids UV radiation cure coatings chemist for three years and have two patents in polymer science).  

An example of incompatibility would be alcohol in urethane - the water can ruin it.  Or lacquer thinner in epoxy - it will work but fogging can take place.  Denatured alcohol is best to thin epoxy, but only less than 10%.  Great mix to fill and solidify wood for topcoating with 2K urethane, although polyester is most common, cheaper, but it shrinks more telegraphing wood grain as the MEK peroxide does not fully extinguish, and continues shrinking over time.

Best cheap, all around thinner, cleaner, etc is a 50% mix of toluene and acetone.  I buy it in 5 gal buckets at a fraction of the cost of "lacquer thinner".  

I quoted this post to my boss at the small furniture company I work at. It's genuinely useful information, and not used as often as it should be. The epoxy plus 10% alcohol....makes life with the stuff a luxury rather than a nightmare. It sounds like you have a lot of extremely useful and practical information to share!

Agreed about the DIY thinners mix. Toluene is a nice solvent to have around the shop since it's good for removing dried glue from wood....

Reason I do not like the work finish wood. Substances not nice and not smelling good. Make me ill toluene.