You can use a grinder to get the bulk. Then switch over to a block and sand paper with 60 grit.
Block and sand steel plate sounds like a lot of work to me.
Go over the welds the first time with a grinder. I like using a 36 grit flap type wheel. It removes fast. Once the welds are getting close to the level of the base metal quit. Now go to a red sanding disc. I like the 80-120 grit to start. Get the weld down to where you are just starting to get scratches in the base metal. Now quit that. Look over the welds real good now. If you have any pits or low spots run another bead over those areas and re-grind them. Once you get everything looking pretty good get a 220 sanding disc and finish the area. Feather a couple inches into the base metal. Very small pits and the 220 scratches will fill with primer. And before you call it done do spray the job with some primer so you can "see" how you did better. Where most jobs like this get screwed up is when you get too aggresive with the grinder and cut into the base metal. Once you get everything smoothed it will look wavy if the base metal was cut into very much. And of course, make sure you bevel the 1/4" stuff well before welding.
BTW I presume you have a A/C only machine? Try some of that 7018 AC rod. Really nice stuff. Very high quailty smooth welds.
2006 Solar Yellow Rubicon Unlimited
Jeep Club Member #1340
6 Speed, Hardtop
Mods done: Hurst tee handle, cheap hand throttle, Rokmen Merc front bumper, Warn 9.5 TI winch with 3/8" X 100 worth of Viking yellow rope, DPG OME Ultimate with JKS ACOS up front, Kilby Gas Tank Skid, Kilby Steering Box Skid, Jeep Medic Belly Up, Skidrow Engine Skid, Rockcrusher Diff Skid in the rear, Warn Diff cover in front, , AR Outlaw II's and MTR 12:50/15's, Homemade rear Bumper, Cheap Cobra CB, Puma OBA, Sirius Radio, Locker Defeat, Rockhard cage, Rockmen short corners, homemade tire swing/tailgate hinge affair, Airlift air bags on the rear- - - - - - and more to come!