Paint touch-up is a common source of problems on job sites, often because of changes in environmental factors such as temperature.
You can spot and correct problems before they occur, however, by using the tips below.
Temperature changes between coats can cause touch-up coats to be lighter or darker than your last finish coat, especially when you work in temperatures that fall between 40 and 60 degrees.
Typically, touch-up coats will be lighter in color if they are applied when the temperature is 10 degrees cooler than the initial application. If the reverse is true, the touch-up coat will appear darker.
Color differences with warmer temperatures—over 60 degrees—are less predictable. Problems may still occur, but they can also be caused by factors such as faster film drying and/or dry brushing.
How to Avoid
Take care to apply touch-up coats in temperature conditions as close as possible to those during your initial finish coat application. To avoid dealing with wide swings in temperatures, apply paint only when temperatures are above 50 degrees.
Touch-up problems can also occur with porous surfaces, particularly with high-sheen eggshells and higher gloss finishes. Because unprimed porous substrates absorb more of the paint, they leave less of a sheen or gloss.
Always apply a second topcoat to make sure you achieve the look you want. Without this second coat, any subsequent touch-up will be noticeable and affect the sheen or gloss you want to achieve.
Poor color hiding can be mistaken for a problem with touch-up. When unpainted, unprimed surfaces are coated with white or transparent colors, gaps are possible.
With these kinds of surfaces and colors, remember to always apply two topcoats over a primer sealer in order to achieve the best hiding.
How to Avoid
Use a primer on all porous surfaces, to make sure they are sealed properly. When you are making drastic color changes or using a white or jewel tone, use a pigmented primer to augment the hiding of the topcoat.
Different paint types require different amounts of time to finish. As a result, the amount of time between the application of your finish coat and touch-up can affect how well these two coats blend together.
Alkyd paints, for example, sometimes require several weeks to reach their final appearance, while latex paints need much less time.
A good rule of thumb to follow: the higher the sheen and gloss you use, the longer the time you will need to allow your paint to completely dry and set.
Certain paint ingredients are slow to evaporate, and—depending on temperature and humidity conditions—need more time to escape from the film.
How to Avoid
Allow at least two weeks for touch-up coats to dry thoroughly, before judging whether you need to re-apply.
When you use different application methods with the same paint job, touch-up problems can occur.
For example, if you use an airless sprayer or roller for your initial application and apply touch-up with a brush, differences in the paint film's profile may be noticeable.
Roller naps leave an obviously different surface profile than brushes. When light reflects off these profiles, the resulting reflection causes a difference in the sheen or color—which is why different paint formulations use specific ingredients for their flow and leveling properties.
Touch-up marks also can be noticed when there is a significant difference in the film thickness of the touch-up and the initial application. The difference is often visible, particularly with higher-sheen finishes.
How to Avoid
Save some of the paint you use at each job site specifically for touch-up purposes. Remember to use the same types of tools to apply them (such as the same size roller nap or brush).
With airless sprays, spray some of the paint through your gun and store it for later use.
When applying, take care not to use too much paint. You can minimize the difference by lightly feathering out the touch-up with a short nap roller. Take care to never "dry brush" or "dry roll."
Additional Prevention Tips
Make sure that your selected topcoat is the right finish for the job. Long walls with light sources at either end are often difficult to touch-up, as are walls with surface imperfections.
In these cases, use a flatter finish. It will be more forgiving with surface and touch-up flaws.