This is a question we get from people who may have a scrape, scratch or chip, and the size or location makes it a candidate for either approach. We've put together a list of questions to ask yourself to help make the decision easier.
1. What size is the damaged area?
Is the damaged area so big and full of scratches that a brush repair will either a) take too long; or b) look very noticeable?
Answer: If this is multiple scratches or a good size scrape (such as from a concrete pillar that dragged across your fender), then it is definitely going to take a long time to apply the paint by brush. And, its probably going to be a visible repair.
Exception: If this damage is VERY low on the car, such as on a rocker panel, then the visibility is not much of a concern. And if it isn't too big, then you can use a brush, and not spend as much time blending and leveling. The rationale is that this repair is more about protecting against future rust, than invisible repair.
2. Where is the damaged area?
Is the damaged area very visible, such as the top of a fender, high on the door, the back of a side mirror, the hood, etc?
Answer: If this is a very small scratch or chip, you might opt for the brush. You can achieve a near invisible repair if you are careful to polish away excess paint, and level the clear. However, if the damage is moderate to large in size or complexity (such as a concrete scrape or a long scratch), then spraying will yield a more invisible repair.
One determining factor is: how big an area do you have to spray? This is a critical factor, because the area you mask off can make a big difference in how the repair blends with the rest of the factory paint. Think of it this way: if you are spraying a mirror, or a gas cap cover, then you have a well defined area to spray, and there is no blending challenge. But if you are spraying the top of a door, then you may want to mask off at the (horizontal) mid line of the door, if it has a molding or a crease in the body work. The reason is that the horizontal "line" in the door makes a good place to end your repair, since the line of new paint is less obvious because of this break in the door panel. You might spray the whole width of the panel, so you don't have to blend the new paint into anything. For other locations, you want to look for good masking regions that limit how much you have to paint, by using parts of the car to help avoid blending new paint in the middle of a panel. It is most difficult to blend new paint with factory paint in the middle of an unobstructed surface, like the hood, or top of a fender. Rocker panels, mirrors, gas cap covers, and bumpers are all good candidates to spray. Doors depend on where the damage is. Hoods are tricky -- it is harder to blend a small spray area, so you might want to paint the whole hood.
Tip: Even if you have a good defining area to mask, you generally don't want to spray right up to the masking tape around your repair. Instead, try to blend it by fading out the spray from the repair area out. If the size of your repair is small, and the fan pattern of the aerosol is too large, then you can cut out a hole in a piece of cardboard, and spray through that hole, to effectively make the fan size smaller. The aperture of the opening in the cardboard gives you better control over the overspray. Its worth practicing that on a big piece of foam core, so you can clearly see the size of your spray pattern. There are some good YouTube videos on this technique also.
Recommendation: Brush or Aerosol
3. What is the damaged area?
For some repairs, the choice to spray is obvious. For example, a mirror, gas cap, motorcycle tank, side panel or wheel fender, a helmet -- all are modest size, and don't require any blending with factory paint. They can be removed or the surrounding area masked, and sprayed with aerosol paint without worrying about blending the paint or clear coat to the factory finish. Below is an example of a gas cap on a 2017 Jaguar F Pace that was keyed, and sprayed with ScratchesHappen aerosol paint and clear coat.