While you could paint your guitar without sanding, there is a high likelihood that the paint you apply could end up falling off.
As a result, it is better to do some preparation work to maximize the chance that the paint you are going to apply sticks to the guitar.
There are three main reasons you should sand your guitar before painting. These are explored in turn.
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Paint Does Not Stick To Glossy Guitar Surfaces
If your guitar already has some kind of paint job on it, then you need to at least scuff up the current paint job before applying a new coat of paint.
The main reason for this is that if you have a pre-existing paint job on your guitar, then it is likely to be a glossy finish, and a new coat of paint will not stick well to an existing glossy finish.
To scruff up an existing guitar paint job you could use 000 steel wool or very light sandpaper. It is important not to use anything that is to course, as this will leave marks on your guitar that could then be visible after you have applied your paint.
You just want something that is going to disrupt the glossy finish on your guitar, so that your new paint has something to grab onto when you spray it onto the surface.
The Paint Reacts Against The Existing Guitar Finish
Not all paint and guitar finish chemicals and compounds play nicely with each other.
Depending on the chemical properties of the different surfaces you are painting on, and the paint you want to apply, they may react against each other and your new paint could flake off.
As a result, it is important to find out the nature of the current paint and or finish on your guitar, and find out whether the paint you want to apply will stick to that type of paint or finish, or reacted and flake off from it.
If you find out that your new paint is not likely to stick then you are going to need to completely remove the existing paint, or apply a primer that will cover your existing paint or finish, before putting on your new guitar paint.
A lot of guitars will have either a polyester, or polyurethane-based finish, but only SOME paint and lacquers will place nicely with each of these.
To Get A Smooth Guitar Finish
The final reason to sand your guitar before you paint it is to make sure you get a very smooth finish.
If you just apply paint without sanding it beforehand, you are likely to have quite a few blemishes, marks, and potentially even grain indicators, that then show through your final paint job.
Potentially, if you apply multiple coats of paint, then you may be able to cover up these marks and blemishes, but be aware that thicker paint jobs on a guitar, do affect the guitar tone.
So it is not good to rely on the thickness of the paint to give yourself a nice smooth finish. You should do the work of filling the gaps with a gap filler, sanding it back nice and smooth, and then applying your paint to a surface that is already smoothed and free from marks.