Flap Discs versus grinding wheels

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I get asked this question by alot of fellow welders/fabricators; which should I use for grinding a weld, a grinding wheel or flap disc. My opinion is that if surface finish is not a concern use a grinding wheel (it is cheaper). However, if the part is going to be painted or is stainless steel use the flap discs which provides a much better surface finish. The grinding wheel tends to gouge/scratch the metal and will show through when painting, plating or on stainless.What do you all think?By the way, if you use flap discs I have found a great source at Lehigh Valley Abrasives (www.lehighvalleyabrasives.com) $1.72 each for Zirconia flap discs that perform well and last a long time.
Reply:For me, it depends entirely on the intended purpose and size of the material to be removed.
Reply:If I need a good finish, after grinding down the weld I use a sanding disc to blend it in and I would imagine that they are a lot cheaper than a flap disc.  But the stuff I work on rarely gets painted or coated. Just buffed up.
Reply:For me it all depends on the size of the weld to be ground down.  If there is enough weld, I use a grinding wheel to take it down close to the base metal then I hit it was a flap disk.  I've found that trying to take the weld down with a flap disk along will leave a slight hump because you're not only hitting the weld, but also the surrounding meterial because the disk is so flexable, where as the grinding disk can be aimed exactly where you need it.Common sense in an uncommon degree is what the world calls wisdom.
Reply:Yep, what Brainfarth said.  Flap disks take more adjoining metal than the wheels do.  I use flaps when I want it smooth on something rounded or smooth to begin with.  But, with that said, I tend to use whatever I left on the grinder nearest me first.Smithboy...if it ain't broke, you ain't tryin'.
Reply:I use the blue sanding disc's for most work, 50 grit.  Usually if I'm using a hard grinding wheel it's for back grinding.  Only thing I use flap discs for is smoothing out grind marks.  I also like 3M Green Coprs grinding wheels.  Seem to have faster metal removal than regular black wheels, and don't gouge the metal as much.
Reply:I like the green sanding disk. Gets a weld smooth pretty quick, as long as its not a massive mountain of metal
Reply:If it's a mountain of metal, sanding disks are not the abrasive to use due to their cost and the fact they fall apart quickly when you heat them up from heavy sanding.Common sense in an uncommon degree is what the world calls wisdom.
Reply:Yes, flap discs can do some strange things when working with ridges and edges and the like. They can pull metal from places you don't want metal removed without seeing any indication that it's happening. If you are using them for major weld bead removal on already thin material you can thin it out too much without realizing it. I use the heck out of them but I get everything close first then finish out with a flap disc.