Just started learning to weld . So welded in rear lamp panel then when i was grinding my welds flush just seem to fall apart and leave holes ?
Try to be less aggresive on making the welds flush especially on the outside of a convex section as it is easy to go 1mm too deep and cut through the panel. You will need filler anyway. Also you may need to cut back further initially to thicker metal before welding in the new panel - your weld blobs may only be really attached to the new bit.
I'd suggest practicing on a test piece similar to what you are trying to weld (same thickness/type of joint). You can then dial in the welder settings to make sure you are getting good penetration and the pieces are fusing properly before trying to weld the actual workpiece. Also make sure that your metal is clean. I'm assuming you are using MIG? MIG hates dirt.
