We like painting. It puts the finishing touch on a woodworking project, or in refinishing a room to add a splash of new color to the walls. The effects are immediate, and depending on the type of paint we choose, the use of the refinished room can be almost immediate.
We don’t mind using a brush, either. We’ve spent our time on the workpiece to join the pieces and prepare it for finish, or we’ve patched the wall and sanded it to prepare to receive the paint. Now, we just want to relax a little, something quiet and quick, and see those immediate results. I guess what we want to say is painting is soothing and marks the conclusion of the project.
When it comes to the walls in a room, though, or lots of cabinet doors or larger workpieces, perhaps a brush is a little too slow and painstaking for us. So we turn instead to paint sprayers and the power of a spray gun. Talk about instant gratification – that wall gets coated quickly, that room gets its new splash of color, and before you know it, the furniture is being moved back in and curtains are hung.
Types of Paint
We have two categories of paints to choose from: oil-based paints, called alkyds, and water-based paints, also known as latex paints. Oil-based paints use a petroleum solvent as their main ingredient to deliver color, while water-based paints use, well, water, as that medium.
It used to be that if you were going to spray, you used an oil-based paint because it was thinner than water-based paint and didn’t clog the spray line or gun nozzle. It took and still takes longer to dry, but its application with a spray gun was fast, and fast was good on large surfaces.
Today, water-based paints are based upon new synthetic resins that are compatible with water. Their consistency is thinner and they are compatible with the use of a spray gun for application. An airless spray gun or an aerosol spray can deliver this latex paint well. However, they are still too thick for a spray gun nozzle and need something to thin them out for a smoother spray gun application.
Types of Spray Guns
In a past article here we wrote about the HVLP spray gun, and you will find that article here. HVLP (high volume, low pressure) spray guns are the spray guns of choice for professional painters.
The higher volume of paint at a lower pressure gives the painter greater control over the spray for an even spread and minimizing overspray and splatter. More paint goes where you want it to go, and waste is reduced.
An HVLP spray gun will deliver upwards of 65% of the paint to the target, which is more efficient than a conventional spray gun. Unlike a conventional spray gun, the line of an HVLP system won’t need to be cleaned after use, so done is done with the painting when you use an HVLP system.
HVLP systems atomize the paint. The paint line feeds the paint to pass by the air stream of the spray gun, and the paint is broken down into small particles that then pass through the nozzle onto the surface being painted. With the ability to adjust pressure and airflow, you have maximum control of the process and the paint’s application.
Thinning Latex Paint
Of course, you want the paint to flow freely and easily when spraying, and we’ve already discussed the advantages of using an HVLP system. We mentioned already, too, that latex paints are thicker than oil-based paints.
Painters prefer using latex paints today for interior work because it dries much faster. In fact, if you are painting four walls in a room, the first wall may very well be dry enough for a second coat by the time you have finished the fourth wall.
Not all paints are the same. They differ, even slightly, in chemical and compound composition, and have different viscosities. These differences can clog the spray gun nozzle and will affect both the spray during application and the drying time after.
Thus, the need to thin latex paints when using an HVLP spray system. Latex paints are water-based and accept water as a thinner. This is distinguished from oil-based paints where some type of chemical paint thinner is required. These use chemicals like toluene or xylene to dissolve the molecules in oil-based paints.
With latex paints, it’s just water. Water has no viscosity, is clear, and is the standard for assessing viscosity. It is necessary to exercise care when thinning latex paints, though – the more water you use, the more the paint becomes like water. Water does not adhere to anything, and it is possible to thin latex paint enough to make it unusable. But, with more paint added, its adherability can be restored.
Keep in mind, too, that thinning latex paint means you have more paint to work with, assuming the ratio of water added is proper. Manufacturers include thinning ratios on the labels of latex paints to help you thin the paint properly.
The process is both simple and intuitive, and if you follow it well, the result is a good viscosity that won’t clog the spray gun nozzle and will give good coverage to the surface being painted.
Here’s what you will need:
- A bucket. Choose a 5-gallon size, even though you won’t be thinning 5 gallons of paint. You want to be able to give it a good stir without spilling.
- The paint.
- Water. Keep a good quantity of water handy, even though you might not use all of it.
- A funnel. You can use it to measure the ease of flow after diluting the paint. It’s a quick visual cue after mixing the water into the paint thoroughly.
- Your HVLP spray system.
- A test surface of some sort. Maybe a piece of cardboard.
Pour some paint into the large bucket. How much paint you add will depend on the area to be painted. If it’s a full room of walls, perhaps a gallon is what you anticipate using.
In researching for this piece, we found a variety of suggestions for the amount of water to add to a gallon of latex paint. One source recommended ½ cup of water to one gallon of paint, while another used a 10% water ratio to the volume of paint. This is where the funnel and the test surface come in handy.
We did some quick calculations of our own, too. There are 16 cups in a gallon, making a 10% solution 1.6 cups. Since you’ll be doing both the funnel test and the spray test, we recommend going with a half-cup of water to a gallon, stirring the paint thoroughly, and running the funnel test. If the flow is still slow and the paint is too thick, you can add more water.
When the paint is flowing freely through the funnel, load a little in the HVLP system and give it a test on that piece of cardboard. When you find the right consistency for both a smooth flow and good adherence, you are ready.
Remember that adding water will change the color of the paint and change its drying time. You should also expect the surface will need more coats of paint to get that finish you desire.
Here’s a video about thinning latex paint for use with an HVLP spray gun. The fellow is obviously not a baker where exact measurements are necessary for a good cake – he is eyeballing the ratio. But, he does show you the texture and thinness you want to achieve with water, so it’s worth a watch.
A few words about an alternative you might consider before we close out…
If you want a smooth finish, you could consider using an airless paint sprayer. Latex paint does not need to be thinned for an airless paint sprayer, and that will produce a slightly smoother finish.
But, professional painters will most often choose an HVLP spray system and thin their latex paint accordingly. The paint gets stretched just a little more, and yes, an additional coat might be necessary, but HVLP systems are convenient, adjustable to give you greater control over the spray, more efficient in their paint delivery to the surface being painted, and don’t need the nozzle cleaned after each use.
Finally, be sure to wear a respirator when spray painting, and cover anything around the workpiece where paint droplets may land.