The theory, as far as I am told, is that paper is too absorbent for acrylic paint, so it makes it dry too fast and prevents blending.
But is it so?
I'll explain how this question came up.
Anyone who's already got the answer, please feel free to skip the explanation and go to the last paragraph.
Complete beginner in acrylic painting, I bought some acrylic paper (290 gsm), some student-grade paint and paintbrushes from an arts & crafts shop, and gave it a go.
This may make you laugh, but what I attempted first thing was blending (I know...), with a fairly large (probably 1 cm or so) flat hog bristles brush (yep), using ultramarine blue and lemon yellow.
You can imagine the streak-fest that ensued, with no indication of blending whatsoever. The paintbrush is now permanently stained blue.
Even just trying to paint normally, I couldn't go over a previously made brushstroke without stripping parts of it off. And I wasn't really applying any crazy pressure.
So I was wondering: how would one be able to blend by drawing the paintbrush over paint that is already there, if it lifts it off instead of mixing the colours?
So, back to the drawing board :), I watched many YouTube videos on acrylic painting, e.g. those by Art Sherpa + others, and learned a lot, including the fact that I needed possibly even heavier paper, and that my paper needed to be primed, despite being marked as 'for acrylic'.
I tried two priming techniques I found: A) acrylic medium with 3 coats of gesso on top, B) 3 coats of white acrylic straight on paper.
This was fairly painful, to be honest. Not only it took ages, but despite taping down the paper, it also really warped a lot, and only after several days it flattened a bit, still bumpy though.
I would not be very motivated to paint if I had to go through this each time.
In the meantime I also bought (what I am told is) better paint (among others, some Talens Amsterdam, not the Expert one, just the basic one for now; some Winsor & Newton Galeria is on its way today).
Not sure if it was thanks to the better paint or to the priming, or a combination of the two, but at least I was now able to dilute the colours more and paint smoother, longer strokes without seeing much lifting of previously made ones.
Still no joy with blending though. It just was not possible to drag the paintbrush in the area between two colours and have them mix on the canvas (what I now know is called 'wet on wet blending').
I still made a few paintings just to try (e.g. this one, this one and this one).
Yes, I know I am crazy, I went for paintings with the highest possible amount of blending required, and two of them were even meant for oil in the first place. Pushing myself? Or just plain silly?
Unsurprisingly, the choppy, blocky look of the sky and sea really ruins them, so I would still be able to find a way to blend.
And here's the big surprise. I spotted this video where the painter shows she makes preparatory sketches in a simple light paper sketchbook. I thought, hold on, I have a sketchbook, a 120 gsm with nothing special to it. Had some leftover primary blue and titanium white from a session I had just finished. Took a flat 7 mm synthetic flat brush, put down some blue next to some white, and: 1) absolutely smooth brushstrokes, no sign of paint lifting off or breaking; 2) when I tried blending horizontally, I got the smoothest, most seamless gradient of blue; 3) no taping, just clamped the paper down, and it almost did not warp at all!
I then went on to paint a dawn sky on another page of the sketchbook, same story, absolutely smooth and much more effective than any heavier paper, primed or not.
I saw this thread where priming is mentioned, and it seems to me that it is assumed that priming is necessary.
So I am now left with some doubts: do I need to prime paper at all? As it's apparently not helping me blend acrylics better, am I doing it wrong? If I gave up priming paper and painted straight on acrylic/watercolour 290+ gsm, or lighter, would my paintings deteriorate with time?
I'm obviously no Caravaggio, but I'd still like them to last a while.