Ever noticed how many farm welds on equipment look snaggy after a year or two? I'm not talking about the actual weld. Even beautiful MIG welds get rough looking after they weather, even if it had implement paint on it after it cooled.
I've found that, just as prep work is the secret to nice welds, prep work is the key to painting over welds. Take time to use paint thinner or other solvent to wipe away all the soot around the weld, then use sandpaper to smooth the edges of the burned paint or the abrupt edges where you had to grind the metal. If you're really fussy, use a buffing pad to remove the grinder marks. Then spray a coat of primer, allow it to dry, then apply a couple coats of machinery paint.
The secret to making the paint job look "factory" is to remove the soot and welder-grime. That soot has an oily texture that prevents paint from properly adhering.
Have I ever just sprayed a coat of machinery paint on a fresh weld, so fresh that the paint bubbled and discolored from the heat? Sure. But if I have time, and the weld is someplace where I'm going to have to look at it often, it's a matter of pride to make it look "pretty."