Oil or Latex? How to Test an Unknown Paint

Read through almost any website or magazine aimed at the do-it-yourselfer, and you’re almost certain to find someone gushing that a fresh coat of paint can completely change the look of your room, house, cabinets, furniture… just about anything but the family minivan. Come to think of it, there’s probably a website somewhere with instructions on how to update the look of your wheels with a fresh coat of paint.

If you haven’t figured it out by now, you need to know that there are two basic kinds of paint – whether you’re going to paint your walls, your siding, or that old dresser you picked up at a yard sale. The two kinds are latex and oil-based, and sometimes they just don’t play well together.

While it’s possible to paint over oil paint with latex and vice versa, doing so could require additional prep work or even adding a coat of primer. It’s usually easier to just match the paint composition to what’s already there. If you need to buy paint for touch-ups or to repaint some trim, it’s almost essential to match oil to oil and latex to latex. If you’re not the original painter, though, how do you tell? Believe it or not, it’s actually pretty easy. 

Here’s what you’ll need:

• Rags or cotton balls, in a color that contrasts with the paint you plan to test
• Denatured alcohol or acetone (you can use ordinary nail-polish remover)

Our Test Procedure

We have a known paint and an unknown paint for our test. The blue paint in the background of the picture above is latex-based (we know this because we're the ones who painted it). A wall-mounted cabinet, painted white, is our unknown, because it was already painted when we moved into this house (everything – I mean EVERYthing – was white.

We start by washing an unobtrusive spot, such as underneath the cabinet or the back of a door, with soap and water and allowing it to dry.  First, we’ll test the blue paint.

Next, we'll soak a little solvent – some nail polish remover or denatured alcohol – into the white cotton swab, and rub it across our clean spot. Now, we check the swab.

As you can see, some color rubbed off when we rubbed the solvent on the paint. That means it’s latex-based. If you look closely at the surface you rubbed, you can see that it’s just a little dull and tacky, though this will disappear in an hour or so – that’s another indicator that it’s latex paint.

Now, test the white cabinet. We used a dark blue washcloth, soaking a corner (yellow arrow) with our denatured alcohol. Then we rubbed it vigorously on a cleaned spot on the white painted cabinet. The results? Nothing happened: that’s an indicator that the paint is oil-based, alkyd if it’s less than about fifty years old, linseed oil-based if it’s really old.

Now you’re ready for the hard part: painting…