I'm not really good at making a car straight as an arrow anymore. My hands have gotten bad over the years so I have to use guide coat over my bodywork to make sure it's straight. I use Evercoat mud and glaze and works good for me.
Just a helping hand on doing your sanding mud. Don't let it get to hard but sand it when it's a bit green. Not to the point that clogs your sandpaper and not be able to clean it but there is a sweet spot in drying time and if you get it at the right time it takes a lot of hard work out of sanding mud. Get it close with sanding it a bit green then let it get dry and finish sanding it. Save a heck of a lot of time and effort.