Flap disk ... NO, NO,NO. The weld is usually harder than the surrounding metal. The flap disk has too much flex and will remove the softer surrounding metal more than the weld - so you end up with thinner base metal and thicker weld. To grind welds you want to use a solid composite disk OR if you're using an air angle grinder you can use the small 2" or 3" disks on the appropriate mandrel.
There is nothing wrong with using a flap disk. If you angle the flap disk just on the welds that will work fine and then finish as someone stated with an 90 degree angle tool with the the disks that were pictured.
