It would make it easier to cut the dash out all along the seams so you can get a good angle on the drill. I didn't actually do a dash i did the front cowl, roof, and pillars, the Cowl attaches to the dash, but its really the same methods involved. Cut away as much dash as you need to to get at the spots easily with what ever tool your using. Make sure you get the spots cut deep enough but not through the other side. Usually you'll see clean metal chips then they will get powered rust on them and that's when your at the next layer of metal and need to stop. Use a small cutter just big enough to cover the spot. Don't use a 3/8" cuter on a 1/4" spot. Your cutting just enough metal to release the spot or make it very easy to pop it with the seam buster without distorting the mating flange.
Just because the cutter says HSS (High Speed Steel) does not mean you spin it as fast as you can. Slow cutting under 1000 rpms with some type of paste lubricant. Blair equipment makes some very good stuff. It really make the cutter last longer and buying direct from blair its actually cheaper
http://shop.blairequipment.com/Cutte...nt-p/11750.htm