perfectly smooth welds are just sanded, you shouldn't need a secondary filler, maybe a high build primer?
Giant and Felt weld beads have the "stack of dimes" look that people associate with AL welding
Specialized and Low have a flatter bead, you can get it pretty easy by using a smaller filler rod slower movement.
Thats assuming they're even hand made, machine welds are usually constant feed and will be perfectly uniform with no dime look.
I weld, I prefer the flat un-sanded beads like Low and Specialized. Sanding, at least in the racecar/performance welding world, is usually to hide something that didn't get welded great.