The covers of J1 and J4 will be pried up and the area around the pins will be blasted cleaned. To accomplish this much cleaning with ordinary tools would take hours and still not result in the pristine surface needed for accurate soldering. The U8 socket and neighboring parts are usually compromised. If corroded, this group of resistors can shut down the reset and interrupt signals needed to get the MPU running. Most will have to be replaced, even after they were blasted clean.
Resistors are sensitive to corrosion and can display inaccurate values.
It s just common sense to replace them. This close-up of the U8 socket shows that the corrosion has damaged most of the pins in the bottom row. Before the board is bead blasted, it is impossible to determine how much damage there is on the copper traces and pads surrounding the U8 socket. The eyelet shown, just above the bottom ground rail, has been completely consumed by the electrolyte. Before any blasting is begun, all of the pins and remaining solder will be removed from the U8 socket.