Eliminating "Roping" Brush Marks When Painting

Eliminating Brush Strokes
To have paint lay down without brush marks means you must start off with a surface that is smooth and clean. If you are repainting trim that has brush marks already, you’ll need to sand them flat prior to starting your doomed from the start.
To prep the surface start with the mildest [highest] grit paper that will get the job done and work your way down to more aggressive [lower] grits if needed. I start with 220 grit and work down through 180, 150, 100 grits, as needed. The goal here is to sand the surface smooth and flat. I sand the entire surface which ensures the best mechanical bond for the paint to adhere to.
Thinning the Paint
Bill taught me a long time that the secret top laying down oil paint is to pour the paint into a working bucket and thin the paint 5% with paint thinner and Penetrol.
Penetrol is an oil-based additive that helps improve paint adhesion and flow in oil-based paints, primers, and varnish. When thinning the paint, thoroughly stir the paint until you reach the proper viscosity. How do you tell when it's ready?
Here’s a tip:
Pull the paint stick out of the bucket and allow the paint to drip off onto the paint in the bucket. When the paint hits the surface and disappears back [level] into the paint - leaving a level, smooth surface - you’ve reached the proper viscosity and you’re ready to apply paint.
Pro Tip:
If the paint is not coming from a new can strain the paint to eliminate debris.
Applying the Paint
Once the trim is clean, you’ve controlled air dust and thinned the paint you are ready for the secret sauce.
Apply paint to ¼ to 1/3rd of the brush and apply paint to the surface, ensuring complete paint coverage. Working the paint into the surface with back-and-forth strokes until it “feels even.”
Once this is complete, you’re ready for “tipping,” this is the one area that requires some skill and feels, for the paint being applied. The “feel” portion of this process comes from experience and getting your reps in.
Using the tip of the bristles apply a gentle “tip-stroke,” from one end of the board to the opposite end - using one continuous stroke [End-to-End] if possible. you will receive better results if you tip in the same direction.
Once you’ve done this leave that area alone and don’t go back over any paint that is tacking or drying.
Lastly, sand lightly [220 grit] between coats. Following this process will ensure that your paint project will be free of brushstrokes and look professional.