One thing I'm pleased to discover that it IS perfectly possible to paint more than reasonably straight lines freehand using (in my case) Vallejo paints. One thing I'd pass on to anyone wary of doing this refers to whether you're a right handed painter or a left handed one? If you are right handed you can only judge the paint flow on the left hand side, or near-side of the brush and vice versa. This means if (as seen below) you're attempting to do invasion stripes, each bar requires you to turn the model round to enable you to only paint that nearside line. The second tip I give, is make sure the model is resting on your working surface - thus avoiding the 'shake'. I've often toiled with stripes and masking but now I think I'm more at ease doing it and the results... well, that's for you to judge?