I have a whole mess of trim to do and I am wondering what is the best way to avoid brushmarks without taking 2 years to finish. I am using an allmost white BM semi
All paints have different properties when it comes to leveling. Some paints self level and leave a smooth finish no matter how hard you try to screw it up. Others will never level properly no matter how hard you try. One technique that will help with the leveling of paints is to add an extender or conditioner to the paint. Floetrol (Latex) and Penetrol (Oill) can be added to paints to help with their leveling properties. An additional bonus is that these products will also help with coverage.
You will also want to use a proper brush for this job. It is essential that you use a natural bristle or china bristle brush made specifically for fine finishes. You should also invest in a high quality brush such as Purdy. If you use a cheap brush you will have lot of problems.
I recommend using Sherwin Williams Pro-Classic Oil based Semi-gloss for painting trim. This paint is a little thicker than standard paints, but if you add a bit of penetrol it will flow nicely. This combination will almost ensure that your trim is as smooth as glass.