Weight Painting is not working There is another issue. (There is alway an obstacle daily.haha)

This time is Weight Paint. And I have no problems painting on other objects, this is the only object that has issues.

Problems are:


When I paint, it only affects the corner of the box. and it does not read this vertex group. Is it because there are no vertices on this face, it only paints on the vertex?
It paints through the box, it should be only on one side of the box. The box has Solidary and Subdivision and paper material.
How could I prevent the sprinkles from passing through the box?
Thanks for your time to review and help me out! :D
Weight painting gives a strength value to a vertex or to some vertices, it determines the level of influence a group has on this vertex or on those vertices. If there is no vertex it can't paint.

You say that it paints through the object. Maybe you've given your object a Solidify modifier (is it what you mean when you talk about Solidary)? In that case apply the modifier. The other possibility is that you've enabled the 2D Falloff option, deactivate it in the N panel > Tool > Brush > Options.

You don't say anything about these sprinkles and you don't explain what you mean by "passing through the box"... If they are Particles > Hair objects and if you want them to appear only on some parts of the box you need to create a vertex group and select it in the Particles panel > Vertex Groups > Density. If they are Particles > Emitter, maybe you need to choose Faces instead of Volume in the Particles panel > Emission > Source > Emit From.