Ok... the difference between siphon and gravity is gravity lets you use lower pressure to AB with.. which in itself might lessen or get rid of the spiderweb thing... it also lets you get a finer line for detail....
the other causes of spiderwebbing are too thin paint and being too close to the model AND too high pressure... you might try to play around with those to see which will reduce that...
here's some links to some guides for thinning and pressure courtesy of Music City from here:
http://www.craigcentral.com/models/pressure.asp
http://www.craigcentral.com/models/thinning.asp
typically it depends on the paint.. MM acrylics tend to be almost ready for airbrushing (some use them right out of the bottle) I tend to use a 3:2 ratio of paint to thinner on them...
Tamiya has the ratio on the bottle for thinning.,. usually 3:2...
enamels I start at 2:1 paint to thinner then adjust from there...
good luck to ya!!!