The hose is a huge source of dirt.

I was taught by an old school painter on one of the forums (that I actually don't even belong to, but learned from greatly):

Before you shoot all your horizontals (these are areas you have to lean over the car), grab a rag soaked with thinner and pull the last 6 to 8 feet of hose through the rag, maybe a couple times.
Shoot your horizontals, then shoot your verticals.
Then on the next coat before you do horizontals again, soak the rag with thinner and pull the hose through it.
Do this on every coat, make it the routine, and it will come as near to eliminating the problem as anything.

Life was a little more complex for me, because I insisted on laying on the floor and looking directly at the underside of the rockers when I shot them
to ensure full coverage. So I spent alot of time wiping off the hose and myself as well as I could. A rotisserie fixes that.

Another good rule to create is, buy or make a hook, then don't ever consciously let the last 6-8' of hose touch the floor during the whole job (but it will anyway, which is why you wipe it before every coat).
Every time you disconnect the gun, hang the hose on the hook.
Make that the routine also.

I also once saw a younger guy experimenting, he wrapped the last 6' of hose in masking plastic and taped it up.

To try to prevent issues, I started the job with a brand new hose, a nice sublime green knockoff of the flexzilla. By the time it was painted, the hose looked like it had been through a war lol.

Also used the classic motoguard filters (aka garage painters best friend) in the air line. Both the orange bulb at the end and the so-called toilet paper filter.
I drained the tank often, and didn't feel I needed to get too fancy with filtration.