The big boys use polyester filler (yeah, bondo). Use the high quality stuff (yes there is a difference).

The key is that good welds don't require much grinding, and therefore not much filler. Polyester filler has some flexibility as long as it's 1/8" or less thick.

Word of caution. Do not "over grind" the welds, it will weaken the joint. Only a very small amount of surface roughness should be removed. Your comment makes it sound like the welds are really ugly. That may be the sign of a real problem. If they're "bubble gum" welds, it could be that there is not proper penetration............meaning it's a bad weld. Flexing of the frame will very likely break the weld even without the grinding if it doesn't have good penetration. A VERY bad situation. Have that checked first before grinding anything away.