What is the ruling on the prayer of one whose hand or one of the limbs of his ablution has many small-sized substances that prevent water from reaching it, such as widespread paint stains, and is there any leniency for him if this is his profession?

The general rule is the obligation to remove anything that prevents water from reaching the skin. However, some scholars have made concessions for a small amount of such a barrier due to the difficulty of avoiding it, and this is the choice of Ibn Taymiyyah. So, whoever's work necessitates the presence of barriers, must strive to remove them as much as possible. If a small amount remains that is difficult to remove, then it is hoped that it will be excused. If a person strives to remove these barriers and something remains that he is unable to remove, then he should wipe over it, and its ruling is the same as that of a splint (جبيرة). However, leaving what one is able to remove is not permissible. If he falls short in removing what he is capable of, his prayer is invalid. As for the prayer of someone whose garment has a large amount of blood on it, it is not valid according to the majority of scholars, unless he is unable to remove the blood, in which case it is excused. What was mentioned about the welder refers to what some Malikis have gone to, which is excusing impurities that affect the clothes of professionals and are related to their work, along with striving to avoid them.