Keeping solder from flowing where you least want it is quite simple,
and neither white-out, nor yellow ocher are the best choice. Everyone
should know that some brands of desktop white-out correction fluid
contain CADMIUM, a toxic and dangerous metal which should NEVER be
heated or used when there is a chance of exposure such as inhalation.
Yellow ocher (or ochre) is a traditional jewelry material, which
when burned, becomes rouge, the polishing material, a type of iron.
Because yellow ocher is iron, it will seriously contaminate pickle
solutions, and otherwise complicate cleaning processes. THE best
choice for a solder stop is the grey gel which Otto Frei, Rio Grande,
and other suppliers sell as “Heat Shield Paste”, or something
This usually comes in a 1 pound white plastic tub about 4 inches
tall Vigor # 54.448) This stuff is great! Smooth, with no abrasive
contents, it can be applied with a finger, with a brush, a stick, or
whatever, and washes off easily. At about $14 a pound, it’s worth
every penny, just don’t let it dry out and that much will probably
last one several years.