Like so many things in life,experience is the route to succesful painting. Stated another way,start with small less valuable items and work your way up to large expensive items. All I can do is give you opinions based on my experience that might help. Good brushes designed for type paint being used is very important. Given a choice,a large number of windows is best painted before install then touched up. Why? Because you have little to no control over humidity and temperature of installed windows. I wouldn't expect to do much sanding on new windows but 1500 grit is insanely fine ,try 180- 220 but no finer than 320. At most I would think wiping with prep-sol or other cleaner is all that is required. Oil base will flow out and lay smoother than latex white oil base isn't as resistant to yellowing as acrylic latex. You are correct in priming,use oil or latex that is compatible with chosen top coat. Other than that,try to work around weather and pratice applying right amount of paint with as few strokes as you can manage. Taping alow's quicker,smoother and longer strokes that result in smoother finish.

Also I would get a better brand of paint. ,such as Ben Moore Satin Impervo waterborne, or Sherwin Williams pro classic, the results will be much better. A good latex primer such as Zinsser 123, or Fresh Start would be a good primer.

For looks it's hard to beat an oil-based enamel. I use the Ben Moore Impervo. It's essential to add Penetrol to restore brushability and flow characteristics to the new formulations. A high quality natural bristle brush will also help.

@Brad,

All paint manufacturer's have various formulations that, for various applications, may or may not work well, esp. when one considers price. Behr is geared more for the DIYer while BMoore & SWilliams compete for contractors with formulations intended to reduce callbacks.

If I'm doing someone else's home, I use BMoore. The higher cost of the paint (even w/ contractor discounts) is worth the confidence that it will go on quickly and well and a callback is not very likely to occur.

If I'm painting my own home or doing a low-budget project, I'm completely satisfied with certain formulations of Behr. Actually, I don't think our house has anything other than Behr (inside or out).

What @klem1 stated above is very true. One mistake many make: passing the brush over paint *that has already started to dry*, in an attempt to smooth-out drips and runs. This is, imo, #1 cause of brush marks. #2 is using a previously used brush (reused brushes are best for primer coats) #3 low quality brush.

"(reused brushes are best for primer coats)"

Kind of,,,maybe so,,,sorta depends,,, but here's the problem with always using new brushes for fine finishing work. We already agreed it require's a good brush to do good work. People will not buy an expensive brush knowing they will only use it once. It take's most diyers several stabs to finish a project,often up to two years. At that rate they will have quite a collection of primer brushes,not to mention money wasted. Brush cleaner is our friend. A good brush will be fine if cleaned well in soapy water or paint thinner after each use but sooner or later paint will dry in heel of brush while on longer than expected break,taking a call or other absent minded incident. When that happens,soak it in brush cleaner to bring it back good as new. Fine brushes don't always make good painters,practice make's good painters. If one is serious about painting,I highly reccomend taking time to practice. Who doesn't have some wrong color or left over paint sitting around? Put it on some scrap boards or paint the dog house so you have a feel for what you are doing before starting on new doors or cabinets.

Use a brush spinner to clean your brushes. I have some brushes that are...I can't even recall how old. Old! But still soft and supple. A spinner cost less than a single good brush, it'll save you clean-up time and money in the long run. And it'll improve your work as your "used" brushes will still be in remarkably good shape.

If you don't paint often, start painting somewhere where a few brush marks won't kill you and finish on your more important windows. As your experience and muscle memory increase, your results will improve. Well, they should improve.