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I am using Liquitex basic Black acrylic. I’m simply pressing the tube and draw thick lines directly on the canvas. No brushing, no palette.

I let it dry but after a few days the result is very shiny and when I pass the finger over it there is a kind of oily layer. I can definitely see it on my finger after touching the “dried” paint but it has no pigment in it.

What is surprising is that I don’t get this with green or red, and I get it less with yellow from the same liquitex collection.

What is wrong with the paint? Could it be linked to the humidity in the air ? (I live in a very humid place)

Vincent Teyssier
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  • Is the paint very old? Acrylic usually dries very quickly and cleanly, so it sounds like this batch may have gone bad... even taking into consideration humidity. Try getting a new tube of the same brand and comparing the behavior. – rebusB Mar 21 '19 at 01:09

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I don't think anything is wrong with your paints. I have experienced it as well at times, even though I rarely work with acrylics.

Acrylic paints go through different stages when drying, slowly forcing all of their 'volatiles' (water and co-solvents) out. But even after drying it will hold a relatively large amount of water, that can still increase and decrease:

a level of incomplete coalescence causes acrylic films to be somewhat porous, leaving channels that run along the walls of the hexagonally deformed particles. These pores are then passageways for water to move in and out of the film.

This occurs more prominently in environments with higher humidity, since the evaporation process will be inhibited. This inhibition might cause water escaping from the paint to remain on the outer film of the paint.
Another possible cause is condensation: the water vapour of the humid environment turns to a liquid state as it meets the water escaping the paint film.


As for the differences in the forming of a wet layer on the skins of the various pigments, I think they might be attributed to them reacting differently to temperatures, or altering drying times. More likely, however, is that the batches have varying proportions of water, pigments and additives.


The quote was taken from this page on justpaint.org. It offers a good overview (of the different stages) of drying acrylic paint.

Joachim
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  • Thanks for this very complete answer. Is there a way to make thick acrylic strokes dry completely? Like varnishing or something alike? I’d like to avoid deformation over time due to temperature or humidity – Vincent Teyssier Jan 23 '19 at 10:14
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    You're welcome. The only way to let it dry out completely and cure, is giving it time and, optimally, a low-humidity and warm environment for the water to easily evaporate and be transported away. Note however, that drying and curing will deform the paint as it loses volume. Varnishing would keep the moisture inside, and will cause problems later on, so I suggest postponing that until you are sure the painting has sufficiently dried. Is there a way for you to lower air humidity? – Joachim Jan 23 '19 at 10:25
  • I can probably reach 60% but difficultly lower. What about torching? – Vincent Teyssier Jan 23 '19 at 10:26
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    As a method for drying, I discourage it, since it will blister and darken, if not burn, the paint - unless you don't mind experimenting with the technique. For drying purposes, I recommend using a (hair) dryer, since it heats up the air, and drives moisture away. – Joachim Jan 23 '19 at 11:31
  • That’s a good advice :) I will test how hair dryer and removal of water inside the paint stroke affect the final shape. Thanks for your advices! – Vincent Teyssier Jan 23 '19 at 13:38