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Why Water Is Not a Pouring Medium
The question we get asked most across the paint counter is why anyone should buy a pouring medium when water is free. It is a fair question with a specific answer, and the answer is about how acrylic paint is built.
What acrylic paint actually is
Acrylic is pigment suspended in an acrylic polymer emulsion. The polymer is the binder: as the water carrier evaporates, the polymer particles coalesce into a continuous film that holds the pigment and sticks it to the surface.
Add water and you dilute both the pigment and the binder. A little is fine — the paint is already water-based. Past roughly a quarter water by volume, there is not enough binder left to form a proper film.
What under-bound paint looks like
- A chalky, powdery surface that rubs off.
- Poor adhesion, particularly on smooth or primed surfaces.
- Colours that dry noticeably duller than they looked wet.
- Cracking and crazing as the film shrinks unevenly.
What a pouring medium does differently
A pouring medium is essentially binder without pigment, formulated to flow. It thins the paint and it adds polymer rather than diluting it, so the film stays sound at consistencies water could never reach.
It also does two things water cannot: it improves levelling, so the surface flattens out rather than holding brush and pour marks, and it extends open time, so the paint stays workable long enough to move.
Our pouring and fluid mediums range lists what each product changes and the bottle volume, which is the practical information.
Ratios, and why nobody agrees on them
Published ratios vary from 1:1 to 1:10 medium to paint, and they are all correct for a particular paint. Heavy body acrylic needs far more medium than a soft body or fluid acrylic, and pigment load varies between colours — titanium white and the earth pigments behave differently from phthalo blues.
Judge by consistency rather than by ratio. The ribbon test: lift some mixed paint on a stirrer and let it fall. It should come off in a continuous ribbon and the ribbon should sink back into the surface within a few seconds. Too thick and it sits on top; too thin and it breaks into drips.
Mixing without whipping in air
Stir slowly and fold rather than beating. Air bubbles trapped in the mix rise during drying and burst, leaving pinholes in the finished surface.
Let a mixed cup stand for a few minutes before pouring so any bubbles can rise. A brief pass with a heat gun or a kitchen torch across the poured surface pops what remains — quick passes, never held in one place.
Choosing a finish
Gloss, matt and satin are properties of the medium, not the paint. The same colours mixed with a gloss medium and a matt medium give visibly different results, and the two can be blended for something in between.
Gloss deepens colour and increases contrast. Matt flattens both and hides surface irregularities, which is useful on a pour that did not level perfectly. Fluid mediums for general thinning and glazing are listed alongside the pouring products.
Common failures and what caused them
Cracking
Usually too much water, a pour that was too thick, or drying too fast in a warm draughty room. Slow drying is your friend — cover the piece loosely while it sets.
Pinholes
Air whipped in during mixing. Stir slower, let it rest, torch the surface briefly.
Muddy colour
Over-manipulation. Every extra tilt blends the layers further. Pours look best when they are moved less than instinct suggests.
Peeling
A surface that was too slick, or one that was not primed. Sand and prime anything glossy before pouring on it.
Drying versus curing
Acrylic is touch-dry in hours and fully cured in weeks. A pour that feels dry the next morning is still soft underneath, and varnishing at that point traps solvent and moisture in the film.
Wait at least a couple of weeks before varnishing a thick pour. Longer in a humid spell.
Where to start
A gloss pouring medium and a small set of fluid acrylics will teach you more in one afternoon than any amount of reading. Our acrylic paint sets and pouring and fluid mediums pages give volumes and what each product does.
We are on 3rd Avenue in Brooklyn, open early Monday to Saturday. Ask us at the counter or by email, see the FAQ, or browse the range.
From our shelves
Products in this store that relate to what is covered above:
Prices are in US dollars. Delivery timings are on our shipping policy page and the return and refund policy sets out the window if something is not right.
Key points
- Thinning acrylic with water past roughly a quarter by volume weakens the paint film.
- A pouring medium thins and extends without breaking the acrylic binder, which is what water does.
- Test flow with a ribbon test: lifted paint should fall in a continuous ribbon that sinks back within a few seconds.
- Mix thoroughly but slowly. Whipped-in air is what produces pinholes in a dried pour.
- Different mediums give different finishes — gloss, matt and satin are properties of the medium, not the paint.
- Let a pour cure properly before varnishing; touch-dry and cured are weeks apart.
