Disadvantages of using a brush to apply single stage paint...?

[Recaro]

It's one of the few things I couldn't find a reason for using Google, so I thought I'd ask in here... forgive me for what could possibly be a super-silly question.

What are the reasons nobody uses a brush anymore, besides the extensive sanding that would surely be required? For fixing a square patch area that was replaced, is there something about the paint, the process, or something I'm not thinking of why I couldn't just put it on in a few coats with a brush, if I'm fine with sanding out the brush marks once I'm done...? Again, this is for single stage paint...

 

[chopolds]

I wouldn't do it, but can think of several reasons not to. Yes, extensive sanding will be needed. You might get brush hairs in the finish. You cannot control the transition to the old paint, i.e., blend it off. When you do go to sand, the edge will be pretty thick, no matter how you try to keep it thin (compared to spraying). So now, when you go to sand it smooth, you will have to sand pretty aggressively, and wind up sanding onto the old paint around the repair, possibly going through to primer. This applies to single stage, solid color only. Don't even think about doing a metallic or pearl.