Classic Car touch up problems

[Steptoe]

Great site, had a good look around how tos while waiting for registartion...
1st..Im no artist, my talent lay elseware in welding restoriing old cars, breeding endangered parrots and gardens...
BackGround:
I drive daily cars I restore rather than modern plastic cars with heaps to break down...A 1st Gen Camaro and a Bradford Van...
These are tidy vechiles, but everynow and then a car show comes up and a few chips in the engine bay/chassis/interior need touch up
These areas I have used a Spraying enamel...sticks to anything and flows well.

I have an elcheapo duel action/syphon feed air brush, a workshop compressor with water/oil filters..(I also do my own car paint jobs, for last 30yrs and pretty good at it)

I thin down the spaying enamel between 50/50 and /75 thinners.
I have experimented with Diff Pressures from 5lb thru to 40lb

I cant get a consistant flow of paint or even straight thinners...air is consistant.

I also have a single action air brush and this does the same...

The air brush is ideal for these touchups, no need for masking etc WHEN it does occassionally does feed the pint consistanly

I have this feeling Im just missing out on something simple...

Any advice hints please?
Cheers
Steps