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Thinning water based paint

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Living here on the wet side of the Big Island (Mt. View gets ~200 in/year of rain) I find that water based paints can take a very long time to dry completely.  Trying to find a solution I tried using my wife's hair dryer to speed the process up and it worked.  Problem was that it took a long time using the hair dryer to get the paint to a satisfactory dry state.  Next was trying something other than water as a thinner.  I got very good results using 91% Isopropyl Alcohol (drug store rubbing alcohol).  Mixed 50/50 with the paint, I was using MSW paints, it made a real difference in the drying time.  Otherwise seemed to work fine with no ill effects.  I am probably not the first to figure this out but thought someone might find my results interesting.

My advice and comments are always worth what you paid for them.

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Hey mate. Acrylic paint dries via 2 main methods; absorption via a porous substrate, or evaporation. By thinning the original paint with any only adds even more liquid to be dispersed via both these methods. If you have access to a speciality paint store they should have drying agents you can apply that will greatly speed up the process with only a few drops. Thinning paint can lead to colour change between area's unless you thin each batch with exactly the same amount of thinner/paint ratio. This occurs because you have changed the viscosity, hence drying time of the paint. A few drops of a professional drying agent is a far better way to go.


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No specialty paint stores around here that deal with retail customers.  Don't really know why the paint dries faster when thinned with alcohol but it does.  I think it is maybe that the alcohol simply evaporates faster under conditions here.

My advice and comments are always worth what you paid for them.

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The owner of a leading airbrush company recommends NOT using alcohol!! It is what they use in there airbrush cleaners to disolve the water based paints.

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  • 2 months later...

We have hardwood floors and use "Minwax Hardwood Floor Restorer" on high traffic areas.  This works well (so far) for me for thinning acrylic paints, and we always have a bottle of it around.  It also seems good as a light sealer.  The viscosity is low and so far no paint has curdled when mixed with it,  good when you are looking for a wash or weathered look.


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Ron is right about alcohol.  One issue with Acrylics is drying too fast - alcohol only intensifies the effect. 


Badger recommends using only distilled water for thinning their Acrylic paints which I use exclusively.  I have never had to thin the Badger paints unless I left a jar open as they are of the proper consistency for airbrushing as supplied.



Kurt Van Dahm






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Where we live in Hawaii the humidity tends to be high so water based paints dry more slowly than they would in a dryer place.  Rubbing alcohol works well for me under these circumstances but may not be a good solution for everyone.  Probably varies depending on the brand of paint as well.

My advice and comments are always worth what you paid for them.

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Rather than a heat gun,  a box with 100- 200-300 W of incandescent lights and a muffin fan or computer cooling fan

would reduce the relative humidity and speed water evaporation.   The light wattage level = the settings to dry faster

but not too fast, and you could do something else as it drys.


I seem remember seeing photos of auto paint rooms with banks of heat lamps -- but that paint is solvent based, so drying rate

there is probably a completely different equation.

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Alcohol has higher vapour pressure than water, hence it evaporates faster ...


What effect alcohol has on acrylic paints depends on the brand. Some brands actually use alcohol in their spray-ready formulations and these are recommended to be thinned with alcohol or an alcohol/water mixture. These paints are quite complex emulsions of acrylic polymers, pigments and solvents. If you change the composition, e.g. by thinning, the emulsions can break down and the acrylic polymers and pigments might curdle - in the worst case they may clog the airbrush or at least you might get an uneven coverage.


The 'drying' of acrylic paints is a combination of processes, in addition to the already mentioned evaporation and sucking up of solvent into a porous substrate, the acrylic polymers slowly beginn to develop physico-chemical cross-links between each other. This process can take several weeks. Thus, initially the paint surface is typically quite soft and rubbery and only hardens with time.



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