The best solution is to get rid of the pin holes with weld. Grind around them so you got a good smooth surface then weld up the pin holes . What I do is put a light behind the panel I'm welding so I can see the light shining thru the pin holes. Another suggestion is to do your grinding as you weld some sections instead of doing all your grinding at the end. This also helps keep the heat down and will help in preventing panel warpage. With no pin holes your plastic body filler will last forever, because there will be no possible way of moisture coming thru from the backside of the panel.
