Will it be particularly painful to get a tattoo over a scar?

No: Please read my recommendation to ur other question about tatto and scars. But tattooing a area with scar should not be more painful, unless there is a problem like active inflammatipn or infection. Then you should avoid the area completely. Even keloids are active inflammation areas and they should be avoided.

NO: If you scars are well healed and without symptoms, use of tatoo needling should not be a problem. The target for most tatoos is the fibrous dermis, which is a lesser version of your scar. Be certain that you are not allergic to pigments being used (test). Many are organic and well tolerated. Scars usually have less nerve supply than normal skin, so often are less sensitive.