Not really picking up what you are laying down but it seems like you are filling one side at a time. That would not work for me. I would put filler over both sides of the line in one pass and try to duplicate the line with the spreader with my last passes running length wise. It takes some practice. I rarely use a spreader bigger than 4 inches. I give the plastic spreader a little curve so I am using one end to dial in the line after the initial application and the other end of the spreader is hard on the panel yet "floating" the line. Gotta think flat all of the time. No up and down (high and low) with the spreader.
Starting on the top (easier to keep flat) file one side of the line letting it run wild but keeping it flat taking in consideration of any length wise curvature of the panel. Always use sharp paper. 36 is pretty aggressive but doable then work it with 80 and then 150 for primer.
Blow it off good. NOW, on the freshly filed side of the line, lay down your tape where you want your line to be. Make sure the tape is stuck well. Take some fast drying rattle can paint and dust it over the tape. This is called a "guide coat". They actually make a spray can called guide coat or there is a powder form that is applies with a foam pad but lacquer rattle can works fine.
Pull the tape and file the other side up to the tape line you created. Hopefully this helps. Sure is nice to get this done in one shot. If you don't get it in one shot, I recommend re-coating the whole thing cause you will never get it right patching it in here and there because of the different hardness of the mixed fillers. You can get away with a small depression if you get on it before it gets real hard.
That's the ticket to most of this. Get on it with #36 when its "cheesy" to start with. It will plug your paper and have a tendency to peel off the panel but be gentle and smack your paper with a paint stick or something to keep it clean. Get it roughed out when its soft then let it dry better and start in with the #80.
