I hate those Blair-style cutters because they leave that little "plug" that you have to grind off later, doubling your work.

I really like the way this style works because it will cut flush to the inner panel (with one or two practice cuts). Usually seeing rust dust is an indicator that you're through!

http://www.ebay.com/itm/3PC-Set-HSCO-Tit...=item51ba0f5e81

These are available at virtually every swapmeet from those guys that sell all those offshore tools.

Unfortunately I found the smaller two cutters are useless because they're too small. The larger one is 3/8" which seems to be ideal. The down side is they are China-made and wear out quickly (got one and an half inner fenders cut before it was toast). However there is a US-made cutter available but the name escapes me right now.