So sometimes I get this annoying line (sometiems a buldge) but mostly a faint line where the filing is. What am i doing wrong to get this? Esp since sometimes i get it and sometimes i dont....
Make sure you use a stiff dam, and pipe it 1/4 inch inside the layer. In other words, don't pipe the dam right up against the edge of the cake. Also, don't overfill. Some people put weight on the cake like tiles or books or whatever, but I never do that and I don't have that issue.
Are you crumb coating and chilling before the final coat?
Yup, I've had this problem before. It can be seen in some of my cake pics. I have found that using a stiff dam AND then trimming the sides of the cake evenly around has eliminated this problem. I crumb coat to catch all the crumbs from the trimmed cake sides. HTH!