Well I certainly haven't mastered it yet...I believe it's plastic junkie that I got it from
For gaps l, stretch some sprue to the appropriate size, I dip it in tenax which melts it to the two sides so it looks like a weld seam. Since its plastic, when you sand it down it won't look like putty. I have never been able to sand putty on a significant seam to where it's unnoticeable.