Richard
You have the right idea but do not grind the weld simply use the file, reason you can impregnate stone residue on to the ally and that can cause problem on your run also make sure to wash all the old flux from the tacks right off so you can start the weld from a clean surface ,make sure to use acetone to clean the surfaces inside and out
Here a tip for you........ to get best result for a flat weld and tack, file the two top edges of your panels off, so they are not square edges, they must look something like this V also it's best to have little penetration as possible so you can hammer your weld right out ,what that means you must be quicker on your speed when welding and have the torch with a feather (carbonizing flame)
PS ... VERY IMPORTANT.....do not hammer any tacks or weld... while the flux is on the surface, you will impregnate flux on to the ally ....the painters will hate you for it!!
Peter