Raised edges from masking can be taken care of with 600 - 1000 grit sand paper. Be gentle and take your time, you can knock the edges off pretty easily and not mar the surface or other paint.
Thinning your paint and applying it in several light coats can help.
I just logged in to say how awesome your axis light walker looks. (I dont really like the dots on the allied…) But the paint looks so plain and perfect..so clean… Props to you!
They look great !!!! now just dirty them up with some powders or paints and washes.
Your off to a great start. I think the first piece of advice I can give is that you should pick up a spray gun (aka single action, external mix, siphon feed airbrush). These are basically, very simple airbrushes. They are no good for detail work, but they are grate for laying down flat coats, are hard to mess up, and a cheap. If your already masking then your basically half way there, might as well just save your self some time and pain and just spray.
MiG weathering pigments would help give them a dirty, weathered look, just apply a liquid fixer, add the various coloured powders and then use an old stiff brush to remove excess powder, the harder you brush, the more you take off. They are great for mud, dirt and soot effects.
I do like what you have done with your walkers though, a little wethering and they will be perfect!