Yeppers! Dats da way to do it!Bondo AKA plastic filler is just that, filler. Not made to be an adhesive or a sealer. Use body sealer over primer period. I work at a large collision shop and I can tell you that when repairs are made on any car the seam sealer application can make or break the job. I have had to redo several seam sealer jobs of other techs from not looking correct to wrong product in wrong location to someone just not caring if it leakes or not. Some areas on a car it can be brushed on. in other areas it may have to have a fine line. One trick not to make a mess is to masking tape along the edge on either side of the seam. Lay in the sealer and work it in to the joint, smooth it with a solvent much like you would a household latex caulk with water, then untape andsmooth the edges with a "acid" brush and some solvent. Just depends on what area it's in and how you want it to look. Practice on some scrap.