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.