Why does touching up paint never look seamless, even with the same paint?

It’s well known that it’s virtually impossible to get a partially repainted surface to match the original surface unless you repaint the entire wall. Say you accidentally put a little gouge in your wall. You fill it in with some spackle, sand it lightly, and repaint that just that little spot. Even if you literally use a can of the original paint that was used to paint that wall, if you look at it from certain angles and in a certain light, there will still be a very slight color difference. Why is this?

I tried Googling for info on this, but all the hits pertain to trying to match paint colors when you don’t have the original paint.

The older paint is weathered/faded/slightly dirty.

You can get pretty close using the same paint and an artist brush to feather in the fresh paint over the old. It still isn’t perfect but you usually need to look for it at least.

Yep. Cured paint starts fading almost immediately. And air pollutants don’t help.

Finish texture also affects the way light bounces off the surface differently - a wall that has been rollered can be very different to the patch that has been brushed on.

I was told by a house painter that after you patch paint you get a dry roller and go over the area. It removes that texture difference and feathers the colour so there is no sharp boundary to catch the eye.

I’ve noticed this too. As you’ve noticed, it’s often angle dependent.

I have a touched up spot that looks perfect 90% of the time, but from some angles and times of day there’s a noticeable difference. Looking more closely, the patched area is glossier than the rest of the wall. So if the room lighting is highly diffuse, there’s no obvious difference, but if there’s a bright directional source (like the sun), the difference in reflectivity makes it obvious.

The glossiness of a paint depends on the ratio of binder to pigment and several other factors. I suspect that some of the chemicals in stored paint evaporate or degrade over time, leading to a change in glossiness. The pigment remains the same color but the overall surface finish changes.

Yes, or with a dry brush. It’s not so much exactly matching the color, it’s making sure there are no hard transitions between the new and old paint. And while you may notice it because you did it, nobody else will notice anything at all.

Sheen makes a big difference too. Touching up flat paint is pretty easy, but the glossier it gets the more obvious it is, because the extra thickness and edges reflect the light differently.

Also, latex paint never completely dries. Oh sure, it loses enough solvent and water to polymerize the latex so you can’t wipe it off, at the end of it’s “drying time.” But afterward, its always changing, getting drier, losing trace amounts of water, altering its structure and apperance. That is until it truly dries, by which time, its for all practical purposes, dried up, worn out and chipping or peeling off. During the painted surface’s lifetime, its constantly changing. “Feathering,” with an artist’s paintbrush, is literally an artist, using their skill of hand, and eye, and perspective of light, to mimic, with the new paint and the old painted surface, what the artist wants to see, an gradient of painted surfaces that tricks the eye into thinking the surface is all the same.

True oil paint isn’t like this: when the solvent (gasoline, in the bad old days) is gone, there’s a colored film of vinyl (essentially rubber, also in bad old days) adhered to a surface. You can then crack open the can, and touch that painted area up, and it will dry to exactly the same surface – baring as above, changes caused by pollution, oxidation, sun fading, etc.

Also, latex paint never completely dries. Oh sure, it loses enough solvent and water to polymerize the latex so you can’t wipe it off, at the end of it’s “drying time.” But afterward, its always changing, getting drier, losing trace amounts of water, altering its structure and apperance. That is until it truly dries, by which time, its for all practical purposes, dried up, worn out and chipping or peeling off. During the painted surface’s lifetime, its constantly changing. “Feathering,” with an artist’s paintbrush, is literally an artist, using their skill of hand, and eye, and perspective of light, to mimic, with the new paint and the old painted surface, what the artist wants to see, an gradient of painted surfaces that tricks the eye into thinking the surface is all the same.

True oil paint isn’t like this: when the solvent (gasoline, in the bad old days) is gone, there’s a colored film of vinyl (essentially rubber, also in bad old days) adhered to a surface. You can then crack open the can, and touch that painted area up, and it will dry to exactly the same surface – baring as above, changes caused by pollution, oxidation, sun fading, etc.

What I’ve found works best is to train yourself not to look in the direction of the stain/cover-up. I’ve avoided looking at one particular section of my living room ceiling for going on 6 years now!

I suppose you could paint an exterior with glossy paint but I don’t recall seeing that. You certainly don’t need full gloss on interior walls unless you love see the light show up the slightest imperfections. It’s well known you can add a small amount of glossy paint to flat paint to get a more washable surface without all the trouble of trying to get a smooth coat of glossy. Frankly, if you are not a great painter you should leave full gloss paint to the pros or use something less touchy.

Part of it is that the new paint doesn’t flow into the old paint to make a single surface. When you paint, the wet paint dries as a single sheet. When you paint over dry paint, the new paint will dry on top and will have a distinct edge where it meets the old paint.

Another reason could be the technique of how the paint was applied. If the old paint was applied with a roller and the new paint with a brush, the texture will be different and will reflect light differently.

Another reason is what is being painted over. Different materials will absorb paint differently and result in different textures or sheens in the final product. Bare drywall or repair material will absorb a lot of the paint and will make the spot standout. The original wall may have had primer, which prevented the top coat from being dried out from the drywall.

Yes but a lot of walls (interior and exterior) are painted with eggshell or satin sheens, because they’re more durable and easier to clean than flat, but they’re also a lot harder to touch up. Trim tends to be gloss or at least semigloss because it’s touched a lot more and needs cleaning more often. So really there’s a sliding scale of trade-offs between glossy (durable, easy to clean, hard to touch up) and flat (fragile, hard to clean, easy to touch up). Satin seems to fall about in the middle.