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.