[Modges'66]

Yes, grind down the welds to make sure that they are not high, sand for good adhesion of your filler, spread a coat of filler, knock down the high areas with power tools, then I block the filler by hand to the assumed finish level to assure flatness, then prime with a high-build primer, then block by hand to maintain flatness. A D-A sander can make it seem flat but still have slight waves, but a block is the only way to assure flatness.
Most areas on my cab & doors have been blocked (in primer) 3-4 times, then guide-coated & blocked.
In my opinion, 90% of the paint job is preparation. if the prep is good, you can just about color-sand & rub anything to make it look spectacular!