what product to use next

Grind the welds down but not too far as you butt welded it, recess the areas that need it, hammer and dolly the metal where needed. Here is what I like to use, been using it for about 15 years now and it is a great filler.

http://www.handsontools.com/USC-16060-BASE-COATCLEAR-COAT-Extra-Body-Filler-Gallon-4cs_p_10764.html