There are many factors that go into a successful cover-up, and many important things to keep in mind when deciding to have a tattoo covered up:
Not every tattoo can be flawlessly covered; whether a tattoo can be covered up or not depends primarily on how complex and dark the original tattoo is.
The majority of cover-ups end up being significantly bigger and darker than the original tattoo in order to conceal the design underneath.
Dark colors in the original tattoo can be covered with other dark, vibrant colors, but cannot be covered with white or other light colors.
If the original tattoo is very dark and cover-up options are limited, it may be beneficial to consider a few sessions of laser removal before having it covered.
When tattooing over scarred/raised lines, they will still be visible through the new tattoo, although less noticeable.
Cover-ups generally take longer and can be more expensive than other tattoos.
scar cover-ups
There are also many important things to keep in mind when covering scars with tattoos:
Anything more than a minor scar must be a minimum of a year old and fully healed to be covered with a tattoo. Scars that are still pink are still healing and cannot be tattooed over. Typically most deep, puncture, or surgical wounds would need at least 2 years of healing before they could safely and effectively be tattooed over.
Scar tissue can be very delicate and therefore takes longer to tattoo over & can take longer to heal.
Scar tissue can also be more sensitive than normal skin and may be more painful.
Some severe or very deep scars may not be a good candidate for a tattoo regardless of how long they have been healed; if the skin is too damaged, the scar tissue can split open. Scar tissue can often take years or longer to fully heal beneath the skin.
Many scars will still be visible under the tattoo, although less noticeable.