Now this is my question: Is it ok to paint acrylic on top of ink??

yes it is absolutely fine.

And I feel reasonably confident that painting oil on top of ink is safe

this is also fine.

I’m working on a canvas that was pre primed already when I bought it from the art shop. Suitable for acrylic or oil. Is there any potential problem applying ink to such a canvas.

no problem at all.

According to Golden, you are breaking “fat over lean rule” using oil paint on top of acrylic.

This doesn’t make any sense- there are some who say not to put oil over acrylic but it has nothing to do with fat over lean…acrylic is always “lean” as there is no fat (oil) in it to begin with. You may have delamination/adhesion issues but you probably will not. Many people use acrylic under oil with no problem.

Thanks for the clarification, and yes that does make sense. I would agree that an acrylic paint film is definitely more flexible than an oil paint film.

Acrylic, however (correct me if I am wrong?) does not expand and contract in a prolonged drying process, is that taken into consideration in the fat over lean reasoning?

“acrylic primings reduced the amount of cracking in the alkyd top layers, appearing to retard their normal brittle behavior.”
So, flexible underlayer can reduce total cracking process into alkyd and oil paint upper layers. Less flexible paint layer over more flexible substrate is recommended by Golden. But “fat over lean” rule recommends to do completely opposite things. To make paint as brittle as possible for underlayer by adding solvent and using low oil content mediums. Something like “Lean paint medium – 1 part oil + 5 parts turpentine”. I am right?
A question is: – which advice is more trusted, Golden recommendation to make more flexible underlayers or “Fat over Lean” recommendation to make less flexible underlayrs? :confused:
I think, Golden idea looks more attractive for me!

But doesn’t it have more to do with how the paint dries? A flexible acrylic underlayer dries rapidly and doesn’t undergo further changes.

Agree. Acrylic dries and set during one month, modern research says six month to set, to shrink and to cross-link between polymer molecules. I took more care about first one month when it can shrink significantly (that depends of acrylic quality)…:) I hope, most canvases in art stores are preprimed and well dried.

Oil paint dries more than 3 years, it is still soft and flexible enough after one year, which is seems to be safe on acrylic gesso.
I am still not sure about using “one part oil +five parts turpentine” mediums. A very risky stuff for me. More or less, I can agree to use 50/50 mixtures.