Touch-ups on walls and ceilings painted long ago can sometimes create a more unsightly finish than the old paint itself. To avoid this it's important to use the original paint, only, for retouching existing finishes.
It's also important to realize that any white will not match any white. There are hundreds of shades of white available from dozens of manufacturers. And what about semi-gloss, eggshell, high-gloss, oil-based, etc. Can these finishes be retouched successfully? As a general rule, only flat paint can be retouched without "flashing".
Flashing is a term used by professionals to describe the obvious difference in finish that will occur when retouching old or shiny paints. Flashing is most obvious when viewing a surface from the side. Look down a wall while standing at one corner of a room, you will see all the imperfections in the surface when light is reflected off of it. Any difference in the overall finish of the wall is called flashing.
Flashing will also occur when a shiny finish is applied over bare, repaired patches. Joint compound and spackle must be primed with a flat latex before finishing with a shiny or oil paint. If you're using flat for the finish it will be self-priming over these patches. If you suspect that the irregular appearance of your walls is caused by failure to prime spackle before applying a shiny finish, coat the spots with flat first and then touch up the shiny finish.
When you have a flashing problem with your touch-up job, the best solution is to do a whole wall from one break to another. This "Paint from Break to Break" method will diminish the noticeable difference in the new finish if the color isn't exactly right.
A break in a wall is usually at the corner but can also occur when trim molding stretches from floor to ceiling, or some other feature of the room blocks off a section of wall. When a wall is broken into isolated sections like this, it can be repainted with a closely matching color and the minor difference will not be noticeable in the overall appearance of the room. Wall corners work well as break points because the two surfaces reflect light differently, making it hard to see varying shades.
If possible, always use the original paint, that is the can leftover from the original job. If the original paint is not available and it is a stock color (a premixed color made at the factory, not mixed at the store) then a new can of the same color will most often work as well.
Custom mixed colors will usually not match the original paint, the problem is the accuracy with which the color is mixed at your local store. Different people, operating the same machine, will use slightly different amounts of tint. All it takes is a tiny change in the amount of tint to change the color enough to show on the old finish. Even computerized machines can produce enough of a difference in the mixed color to cause a mismatch. If you must get a new batch, check the color for matching before proceeding with your retouch job.
Make sure your paint is mixed well before putting it on your walls. Shake the can or stir just before using it and apply it using a brush on small areas and a roller on larger spaces. Check to be sure the new paint is blending well with the original finish before doing the whole job. Do a small area first and wait for it to dry completely, use a hairdryer if you're pressed for time. When it is completely dry, examine the finish while looking down the wall, if you can see the spot where you applied the new paint then your touch-up job will not look very good. If this is the case, use the "break to break" method described above.
Shiny finishes can be the most difficult to touch up. This includes eggshell, semi-gloss and high-gloss, as well as all oil and alkyd-based finishes. Even flat oil and alkyd will flash if you try to touch up in the middle of a wall or ceiling. For all these finishes use the "break to break" method to freshen the finish. Original paint or not, the only way to avoid a flashing problem is to do a whole wall.
Trim should be treated in the same way as shiny finishes, painting from one break point to another. Break points for trim are the points where two separate pieces of molding come together as at the corners of a door or window casing. The corner of an individual piece of trim molding can also form a break point between the face and the edge that contacts the wall or ceiling.
To touch up on a door casing, for example, do all along one side from the floor, to where it meets the header. Don't do the narrow edge of the casing where it meets the wall, just do the face. The touch-up will not be noticeable because the edge is seen at a different angle than the face and reflects light differently making any the difference in color less noticeable.
Touching up the paint is one thing, but stains present another problem. Some stains will bleed through the new coating if they aren't sealed first. Water, ink, cigarette smoke and grease stains will all ruin your touch up job. To avoid this problem use a shellac-based sealer to block these stains before applying any finish paint. It may be necessary to do from one break to another when treating stains like this. See Repair Tips for Stains and Repair Water Stains for more about dealing with this problem.
Shellac sealers dry very quickly (about half an hour) and will block almost any stain with the exception of tannin from redwood trim. Redwood is not commonly found in new homes but historic building will often have redwood trim that has been painted. When tannin from redwood bleeds through the finish, use a water-based sealer like Kilz2® to block the stain and then touch up as described above.