I'm removing the firewall and replacing same. Any ideas as to what size spot weld drill I should use?
Thanks.

http://www.suttontools.com/products/spot-weld-drills-pd142/ 

6.5 mm should do most spots. Cut through one metal thickness, leaving the 'good' panel virtually undamaged.

You can sharpen a normal drill to do the job, as per the bought one. Keep the RPM down.

Thanks Greg. Local shop looks like they stock those.
The 'good' panel will be reused into my bug. Mine is rusted on both upper corners and so far the good one looks ok.

I've used them and find the 8mm is better at fully releasing the spot weld. With the 6mm it doesn't always get the entire weld especially as VW seemed to use an oval or oblong spot weld head on some joins. Rear apron was round spots but heater channel was oblong. Hence the 8mm was enough to cover all the oblong weld

I also bought 5 cobalt off of Ebay. 6 years later and still work fine. Think I've sharpened 2 of the 8mm so far in that time.