You Can't Paint Latex Over Oil ! ...Or Can You?

Using latex over oil based paint has generally been seen as a no-no, and for good reason.  Just taking a quick glance at the picture for this blog post will give you an idea of what will happen every single time you do.  Well...maybe not every time.  

So how do you know when it's safe to use latex paint and when it should be avoided?  That will be the topic of conversation for this week's blog post, but before we get into all of that, let's look at why a homeowner might choose to use latex instead of oil in the first place.

Latex vs. Oil

Oil-Based Paint (sometimes referred to as 'alkyd-based' when synthetic resins are used) generallyadheres very well to surfaces because of it's slower drying time, is extremely durable, as well as stain-resistant.  The drawbacks to oil finishes are that they often have a more pungent odor and higher VOC content, are messier to work with and cleanup, don't retain their color and gloss as well as latex, can "yellow" and even become brittle as they age.

Water-Based/Latex/Acrylic paints are more popular because they're easier to work with and clean-up, they last longer, retain their color and gloss better than oil, usually have a more tolerable odor, and in most cases they dry quicker to the touch.  Latex does have it's cons, in that it lacks the durability of oil in many cases, doesn't generally adhere as well due in part to the quicker drying time, and it shrinks as it dries which can cause previous coatings that are not well-adhered to the surface to prematurely fail.

When Should You Use Oil?

Oil-based paint is a safe choice when painting over a previous oil finish because it is more forgiving of marginal surface prep so there is less chance for peeling -- but that doesn't mean you should skip out on surface prep altogether.

When you want a very smooth painted surface -- such as on interior doors, windows, and trim -- oil is an excellent choice because it has self-leveling properties that will reduce the appearance of brush marks and roller stipple, leaving you with an elegant look.

In applications that call for a very durable finish, as opposed to latex, oil based paints are great at resisting abrasion, scratching, scuffing, and just general abuse.  Outdoor metal furniture, metal handrailing, and other similar surfaces are good examples of situations where homeowners prefer its use.

Oil-based primers for use on bare wood are probably my very favorite application because they provide benefits that no latex product can come close to matching.  If I'm re-painting exterior wood clapboard siding, I'll always have a can or two handy to use as a spot primer because of its remarkable ability to bond to bare wood.

A couple of situations where I try to avoid using oil products are when I'm painting a large interior surface, like walls or ceilings, simply because it's most often not necessary and I don't like to deal with the odor and the cleanup.  I also prefer to steer clear of them when I'm re-painting large exterior surfaces, such as the siding on a home, because I know that it won't hold up as long as an acrylic-latex between re-paints. 

When Can You Use Latex?

It's generally good practice to use latex over previously-painted latex on any surfaces that expand and contract.  Latex paint films are flexible and they move with the surfaces underneath of them, so if I apply a non-flexible coating (which oil is) over a flexible one I'm going to end up with issues that you would expect -- namely cracking, alligatoring, etc.

Latex paints (specifically 100% acrylic paints) are the go-to products for exterior home re-paints because they retain their color and gloss much longer when exposed to the sun.  One situation where caution should be used when re-painting with latex is when it's being applied to an older home that has many layers of paint.  The problem lies in the way that latex dries as opposed to oil.  Oil-based products dry through solvent-evaporation and oxidation, whereas latex dries by solvent-evaporation and coalescence.  Those terms don't necessarily mean much to the average homeowner, but what is important to you to know is that latex paint shrinks as it dries.  So if you have old previous coats of paint that are likely marginally adhered to the substrate -- at least in some spots -- your new latex paint can actually pull them off of the house as it dries.

So How Do You Paint Over Oil With A Latex Paint?

If you've made the decision that a latex topcoat would be best for your situation, what are the steps that should be taken to ensure that it will last?

Clean the surface - As with any painting project, you should remove any dirt, grease, and contaminants from the surface with a good quality degreasing pre-paint cleaner.  If this is the exterior of your home and you plan to pressure-wash the surface, you can use the chemical injector tube on your pressure washer to apply the cleaner to the surface.  Let the surface thoroughly dry before you move on.

Scuff It Up - With a hard coating like an oil-base finish, it is good practice to abrade the surface with sandpaper, or an electric sander, to create a profile for the new paint to stick to.  After you've sanded, be sure to remove the dust from the surface by wiping it down with a damp rag or tack cloth, or rinse it off with a garden hose or pressure washer if the project is a large exterior surface.  Allow it to dry before proceeding to the next step.

Prime It - Even after scuff-sanding the surface it's a good idea to add an extra layer of insurance that the latex will bond to the oil by applying a coat of an adhesion-promoting primer that is appropriate to the situation that you're working in (interior, exterior, etc.)

Apply Two Coats of Your Finish - Using your method of choice (brush and roller or airless sprayer) apply two full coats of your favorite latex topcoat that's appropriate to the situation.  If it's an exterior project, select a high quality 100% acrylic paint with a good warranty.

How Do I Know If It's Oil Or Latex?

A simple test you can do to determine whether you have a latex or an oil finish on your home is to purchase some denatured alcohol, which you can find at your local hardware store.  Apply some to a clean cotton rag and rub it vigorously across the surface.  

Look at your rag.  If you see paint on the cloth that was removed from the surface you should be dealing with a latex.  Denatured alcohol will cut and remove latex but not oil based paint.

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