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How to Make an Acrylic Paint Primer
This article is about the materials and processes used to make a simple, solvent-based, acrylic quick-dry paint primer. The best application is by spraying with an airbrush or paint sprayer—although it could be applied by brush if different solvents are used.
Use Three Processes
The primer is prepared in three simple yet somewhat laborious processes. First, a concentrated (usually 30% weight/volume ratio) solution of an acrylic resin is prepared. The specific resin chosen for this preparation is Paraloid B48N from Talas online. This acrylic resin is an especially hard resin with excellent adhesion to both bare and treated metals. This resin is soluble in strong organic solvents such as xylene, toluene, acetone, and MEK (methyl ethyl ketone). I use a 3:2 blend of xylene and acetone to dissolve this resin.
Prepare Acrylic Resin Solution in Solvent
For the primer, a mix of 30% w/v solution of Paraloid B48N is prepared in a blend of xylene and acetone in a 3:2 ratio. The total volume I chose was 250 mL. To make a 30% weight per volume solution, one needs to calculate the weight of the solid portion. Take 30% of 250 to arrive at a mass of 75 grams. The 250 mL volume will be the total volume of resin dissolved in solvent. So it is recommended to start with a volume of solvent slightly less than required as more solvent can be added later to top off to correct volume.
Polymers like acrylic resin have large molecular weights and take a considerable time to dissolve in an active solvent. Salt and sugar are made of much smaller molecules and they dissolve in water rather quickly. To accommodate this slower dissolution rate:
- Make a satchel of the resin granules with a piece of cheesecloth tied closed at one end with clear fishing line.
- Punch a hole through the lid of the mixing container.
- Feed the fishing line through this hole just until the bottom of the satchel touches the solvent.
- The solvent immediately wets the particles and soon the granules coalesce into a sticky gel that gradually drains into the solvent increasing its viscosity.
- Check periodically until the satchel is empty. This can take 24 to 48 hours. Periodically checking the progress with a little swirling and shaking can help.
Dry Pigment Blend for Primer
The dry primer mix is prepared from equal weights of the following compounds: calcium carbonate (chalk powdered), talc, and titanium dioxide. These three pigments offer beneficial properties to a coating mixture such as tooth, corrosion protection, hiding power, and the ability to be sanded.
- Calcium carbonate, also known as chalk or whiting, is a fine white powder used in gesso and to make chalk paints. Calcium carbonate is slightly alkaline and slows down the rusting of steel. Somewhat translucent when dispersed yet makes paints and finishes easy to be sanded,
- Talc is a white pigment and filler that also slows down corrosion because of its plate-like structure. This pigment helps in the dispersion of other pigments and makes the primer easily sanded.
- Titanium dioxide is a very opaque white pigment that will give paint finishes good hiding power. This is important if you are painting over an existing paint finish and drastically changing the color.
Add Dry Pigment Blend to Resin Solution
- Use a small paint mixer installed into a tabletop drill press.
- While running, a vortex is generated in the liquid mixture, and the pigment is gradually added with a sifter to prevent large clumps of pigments from forming.
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A mini-sifter for applying enamels was used because it was the right size. For this preparation, 15 grams of total pigment was added.
Thorough Stirring Required
After the pigment is sifted into the resin, the entire mixture was stirred for an additional 10 minutes. This was done to prevent agglomerates of pigment from reforming and to keep more of the pigment dispersed throughout the mixture.
Initial Problems With Primer Spray Application
After the mixture sat for several hours, I merely shook the storage jar of the concentrate and then poured some into a spray brush bottle to be thinned and sprayed. I thinned the resin primer with an equal volume of MEK because its evaporation rate is in between that of acetone and xylene. I shook again and sprayed the steel bar shown below. What can be seen here is not only a low pigment concentration but the presence of agglomerates within the white pigments.
Solutions to Spray Application Issues
The first step to remedy the issues with spraying was using motorized mechanical stirring. For the small quantities being tested here, a mini-stirrer for airbrush use can be used. I stirred the primer before thinning moving the stirrer up and down in the mix lifting up denser pigment layers towards the top. I did this for a couple of minutes. I thinned with MEK again 1:1 and added a few grams of white universal tint. This was stirred mechanically until thoroughly mixed. It was immediately sprayed onto the steel bar and a very smooth matte white finish resulted. It dried in less than 10 minutes.
Thoughts and Observations
This was a learning experience but quite enjoyable overall. Initially, I did not add nearly enough pigment mixture to the resin solution, nor did I mix the primer with a mechanical stirrer adequately. I included my mistakes in this preparation to provide assistance to those who might be troubleshooting their process.
This primer does dry very quickly and top coats will be applied at a later date. Three materials were ultimately tested: pressure-treated wood, hot-rolled steel, and 2024 aircraft-grade aluminum sheet.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2019 Jason