looks pretty good, make sure you weld it shut, if you don't feel that confident that it is completly sealed use marine grade fillers as they are waterproff.
DO NOT USE BONDO
unless you want to do this again in 4-5 years. Once i weld all my bare metal I use a product Ernie turned me onto "Zerorust" I paint it by brush with that stuff, let it dry good, then "scuff" it with 80 grit so the filler sticks, then i use a marine fiberglassed re-inforced body hair putty. Basically waterproof filler with glass mat mixed in. This stuff is super hard to sand though so don't go globing it on. Put it over all the weld areas as this is like laying fiberglass over it to seal it. This will ensure no water will come through your paint from the backside again. Once you use that stuff, use a sanding filler (waterproof) or atleast water resistant. depending on your money budget. Sand that with 80 grit to get the major hi's and low's (cross sanding). then hit it with 180 grit, then primer, then 220, 400 paint, clear.
BTW the nasty black guey stuff came stock. It was a problem when i welded up the same area on mikes car (on3go).
-Ed