If I may ask, what brand and type of paint did you use?
AFAIK, Tamiya paints are formulated for use with airbrushes. It takes a very experienced modeler to get a smooth coat of paint using a paint brush using this brand.
Also, white is usually one of the toughest colors to paint.
I recommend a few light coats instead of on heavy coat, if you want a smoother coat of paint--this applies to both airbrushing/spraying and hand painting in general.
BTW, masking does nothing for the paint job if you're painting with a brush. In fact, the capillary effect will actually suck paint under the tape. You're better off touching up the edges free-hand with a fine brush.
There's a trick to smoothing out the paint job after it dries (but not after curing, of course). Apply a THIN coat of thinner (acrylic, enamel or lacquer thinner, depending on what paint you used) on the surface and let it dissolve the paint a bit, and just let it dry again. Once the paint returns to liquid form, the brushstrokes will melt away, and you're left with a smoother coat of paint.
I tried that on an Armored Core kit once, and it works to a certain degree.
DISCLAIMER (so you won;t come hunting after me with a shotgun if something bad happens
): Be
VERY CAREFUL when using this technique
ESPECIALLY WITH LACQUER THINNER, as too much thinner will damage the plastic itself!
Apply as little of it as possible and see how it goes first.
Hope this helps.