Getting a good finish with brush painting is hard regardless of paint.
A quick word on airbrushes though, eBay has lots of cheap gravity fed dual action brushes at a cost almost as cheap as a very good quality paintbrush.
My cheapest one for example, (which came with my cheap compressor), can be had on eBay for 15GBP, currently about 25USD if you're stateside.
It's been a great brush and I've used it for everything from Acrylics to Alclad lacquers.
The compressors are not the cheapest but considering you only buy once they're pretty good value, a compressor and brush kit can be had on eBay for 70GBP, about 115USD.
Re brushing, i'm no expert bear in mind but my observations are:
Humbrol enamels, (i'm assuming you're using enamels?), are great for brush painting, being incredibly thick they need thinning quite a lot, it's hard to say percentages but your looking for a consistency that sticks well to brush and model, flows and covers well and settles within 5-10 seconds, try to avoid brushing over the same area twice, not always possible but this helps it to settle better. Use the proper thinner too, i've tried thinning Humbrol with turps and found it blooms and doesn't give a good finish.
You can dump the thnned paint straight back in the tinlet too for future use, eventually just thin and stir in the tin.
On that note, with all humbrol tinlets, stir them very very thoroughly until you can feel no bits, lumps or gungy stuff in the bottom of the pot, seal and shake the hell out of them then and only then decant, thin and paint.
Tamiya acrylics aren't the best for painting, but can paint quite well, some say they don't require thinning, I find they work better when thinned a little with Tamiya thinner or IPA, decant and thin capping the paint quickly as acrylics dry off quickly.
You'll likely already know but don't try and cover with one coat, this is the biggest problem most brush painters face, with a pale colour like white, yellow etc assume three thin brushed coats to get good coverage.
I hope this helps you.
Just read your other post and noted you're using HUmbrol acrylics, other than the tiny 'kit' pots of these i've not used them and have been grossly unimpressed with them when I have.
It seems like the problem you're having is the same as the one you get with Citadel paints when thinned with IPA, they coagulate, thinned with water should be OK, ideally distilled, this can be gotten cheaply from car spares shops, i'd still recommend humbrol enamels for brush painting.
