Q: any long term effects of adding flow aid & thinner to a bottle of paint vs using it at the time of painting?



several sources of decanting odd shaped bottles into a consistent 17ml dropper bottle suggest using the above additives in order to facilitate transferring and it runs contradictory to what i have always been lead to believe.

https://youtu.be/p_OR2ZeMKNU?si=nGD50H7SgLnuEm-h

It's better to do it on the palette since you can then adjust the ratio of thinner (or diluted flow aid) to paint as you go.
But if you want to have premixed bottles, go for it. Flow aid is just a surfactant and it is already a component of many brands of acrylic paint. As for 'thinner' I assume that you mean something that does not contain any acrylic polymer : follow the manufacturers directions since you do not want to underbind your paints.

i was under the impression paint additives (when added before use & stored) would somehow magically shorten shelf life.

i tend to paint in spurts and then enter a long period where i'll work on a few different projects without needing more than priming.

Long story short. Yes there will be an effect pouring it in the bottle and not using the whole bottle in a short time. It all depends on what name brands you use. If you add glycerin or have already in a paint as a surfactant then you will be in trouble with thinners and reducers, thing will start reacting chemically. Everyone has a good formula that has worked for them for years, probably Will have one of the next few posts. I have more ruined bottles of paint that I can put up against them.

Just use them when you need them. If you feel the paint is too thick in a bottle then only add sparingly distilled water. Nothing else. And I mean period!!!!!!!!!!

Cheers,
Todd