Some artists maintain that a painting should be executed in a single tool type – all brush or all palette knife for example. I don’t think this is true, however the idea behind it – consistency of approach to create the illusion – is definitely a consideration. If you’re using a smorgasbord of tools and techniques, it’s far more difficult to create a consistent image – it can so easily look bitty.
I think it’s important to aim for consistency with the palette knife – propagating effects with similar motions and paint consistencies. A palette knife works well with acrylic as with oil and the key is to generally be using thick paint. Place the paint, modify once (maybe twice) and leave it be – you don’t want to become too fussy or “spread the butter” too thin.
This website has some sound advice (although for oil, it’s mostly the same for acrylic): https://www.artistsnetwork.com/art-mediums/oil-painting/painting-knife-techniques/
I also like this video because the artist plays to the strengths of the technique, using the shape of the knife to effect: https://www.youtube.com/watch?v=k1Jep2ave4s
I personally use the knife for specific passages to create a distinct feel. I regularly take paint off with the palette knife, at times scraping down wet and drying paint – I like the half-cleaned result and sometimes leave it that way. Some painters insist on a hard surface in order to firmly apply the paint without the inherent give of stretched canvas, however I prefer painting on canvas and although the knife may dig in at times, I find it quite manageable.
Break-a-knife!
David