A repaired or repainted area that is noticeably different than the surrounding space.
Applying the touch-up with a tool different from what was used to apply the original coating, for example, touching up a roller application with a brush.
Using a batch of paint that is different from the original application.
If the area was repaired, and the repair has a different texture from the surrounding surface.
Applying the touch-up at a temperature much different from the original application.
Use the same tool to apply the touch-up as was used to apply the original paint.
Try to keep some of the original batch of paint for touching up.
During the repair process, make the repair match in appearance as close as possible to the surrounding area, or feather the repair out into the surrounding area to reduce any abrupt change in texture.
It may be difficult to know the environmental conditions that existed prevalent when the original coating was applied. Use good painting technique practice when applying any coating; follow the label or data page instructions for acceptable environmental conditions.
Some thinning of the touch-up coating may help it blend into the surrounding finish.
Carefully feather the touch-up into the surrounding surface.