When you say the painting disappears do you mean that you paint the arm bone, do something else then when you come back to the arm bone all the weight has disappeared? If so, I wonder if you have an armature modifier active?
If this isn’t waht you mean then tell us your actual workflow and what version of Blender you’re using on what OS?
Manual skinning versus auto-skinning versus weight-painting versus envelopes. Sorry, religious discussions aren’t allowed
In all seriousness - it depends. For something simple like Gus, auto-skinning can be useful but it still needs cleaning up and this can take as long as skinning manually. I use a combination of weight-painting and vertex selection > add to group.
In complicated areas, I select a bone, add a spot of weight-paint then switch to edit mode. The painting creates the vertex group and that group is active when you switch to edit mode. Select verts (loop and box select can speed things up) and ADD to the group. Then tab back to weight-paint mode and repeat for the next bone.