The purpose of this project was to provide data that could be used to support work by Cairns et al.  that aimed to optimise robotic fillet welding by identifying key contributing factors. The focus of this thesis was the effect of weld-through primers on the geometry and quality of a fillet weld. Steel samples were sent to a paint company, where the primer was applied top redetermined thickness ranges. The DFT of the sample plate and a randomly selected panel was then measured and systematically recorded for analysis. Test joints at three DFT ranges, were welded using five filler types and a mechanised welding rig at the University of Strathclyde. Varying test methods were used to identify any trends, that would suggest the coating was impacting the weld quality and/or geometry. Findings from the measurement of DFT casts doubt on data supplied by paint companies. This is a result of the effects of overspray, creating areas with a considerably higher DFT. Weld-through primer and HAZ area, showed a possible relationship. The samples where the paint was mechanically removed seemed in some cases to produce larger HAZ areas. It was concluded that the primer may limit the spread of heat, resulting in a smaller HAZ. The analysis of weld penetration data showed a possible relationship between weld-through primer and weld penetration. The sample joints, where the paint was mechanically removed, have produced welds with less penetration than those coated with primer. Discussion suggests, this could be a result of a surface tension effect from the coating and/or gases from welding, contaminating the gas shield and increasing the arc temperature. This work has also highlighted the potential for a model that would provide manufacturers using weld-through shop primers with a method of estimating internal weld metal porosity by measurement of DFT.