Tony if you see this chaps thread http://www.mig-welding.co.uk/forum/threads/honda-stepwagon-rusty-sills-help.75323/ you will see from his 3rd paragraph that he was not a confident welder BUT follow the thread and see his finished result, should give yo some inspiration. Note when butt welding sheet there is a tendency for the joint to sink and or raise. So when the weld is ground back you end up with the results seen on the top weld of your pic. If you continue to try and grind the weld down you only succeed in thinning the joint (usually indicated by the metal going blue due to it's thinness) Ref you roof welding, you really need to post some pics. If you look at the home page of the forum the top pic is of the sort of weld you would do. Working you way round and cooling lots as you go. Then filling the gaps on a second pass. The technique is to pull trigger to form a spot of weld then as it cools (see the glowing in that pic) you see the hot spot get smaller, as it does you hit it with another spot which will flow into the first spot and so on (using the shrinking glow as a guide) Don't do any more that about a half inch before moving to another area or you will cause warping. Have a spray bottle of water to hand to hit the panel with ever few welds. If you touch the panel it should be cool near by, if hot then damp down before continuing.