The Rule of Thumb 
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First, here's a tip when it comes to sanding paint by hand or machine...
Don't sand where you cannot safely machine compound.
The reason for this is because if you sand right up to an edge or up to a raised body line, it's difficult to run a buffing pad up to the end of your sanding marks and
not buff on top of the body line or edge and risk burning through the topcoat.
Even if you're extremely careful it's all to easy to accidentally run the pad onto the edge and it only takes a
few seconds to generate enough heat and abrasive sanding or cutting action to eat through the paint on a high point and expose the color coat, primer or even sheet metal underneath.
Remember paint will tend to be thinner on high points as wet paint, (when being sprayed), will flow off
a little more than paint sprayed onto a flat panel.
So a good practice is to only sand where you can safely machine compound or another way of saying this is to not sand where you cannot safely machine compound.
The Rule of Thumb
The rule of thumb I use
is with my thumb, mostly because I always have it with me.
I use the distance from the tip of my thumb to the end of my thumbnail, which I just measured and it's about 3/4 of an inch and that's about how far away I stay from edges and raised body lines when I sand by hand or machine.
Actually a 1/2" is a pretty good rule of thumb the bigger idea is to
not sand paint where you cannot safely buff with a wool pad, a cutting compound with a rotary buffer.
This is a technique I teach in our wet-sanding classes to give students a simple method or a
simple guideline they can use to know just how far to sand when sanding near a hard body line or the edge of a panel. This is a simple technique that works for me but feel free to come up with your own method.
Show Car and Perfectionist Sanding and Polishing
For those of you wanting to do
perfectionist sanding and polishing, first make sure you're getting paid well because you can sand right up to an edge but removing your sanding marks has to be done carefully and this means it will require you to
invest more time into the project.
How to sand right up to an edge and remove your sanding marks by hand
There is a fix for the issue of sanding right next to an edge or a raised body line and that's by carefully sanding and then using Meguiar's M105 Ultra Compound by hand to remove your sanding marks. Another way to remove sanding marks next to an edge or to a raised body line is to use Spot Repair Pads on a rotary buffer using a technique I call
Edging.
Because M105 is aggressive, (rated at being able to remove #1200 Grit Sanding Marks on fresh paint), and recommended for use by hand, you can easily remove sanding marks by hand if you're willing to
put a little passion behind the pad.
There are a lot of compounds that can be applied by hand to remove sanding marks, but it's the ability of the
super micro abrasive technology, (SMAT), used in M105 that gives it both its' cutting ability and the ability to polish out clear and glossy that makes M105 Ultra Compound a good choice for carefully rubbing out sanding marks right up to an edge or a raised body line
by hand.
Note: The
blue line you see in the pictures below is vinyl tape applied over the top of a body lines on the hood of this 1969 El Camino to make it easier to see and protect them. You can also do this with the edges of a panel.
In the picture examples above, the raised body lines running down the hood
can be hard to see once you start sanding a panel as you'll have paint slurry all over the panel thus camouflaging the raised body line. Like this, note how it's harder to see where the tape lines are with paint slurry on the panel...
If you don't know exactly where the raised body lines are, it's all to easy to accidentally run your sanding disk, (by machine), or sanding paper, (by hand), over the raised portion of the body line and remove way too much paint way too quickly.
While you might not sand through the clear layer on the body line if you accidentally sand on top of it, remember you still will have to compound and polish the paint and compounding and polishing will also remove some paint; so the risk of sanding or burning through the paint on a raised body line is great.
Here's a tip...
Take a moment to mark any raised body lines with some thin painter's tape and avoid a costly and time-consuming mistake.
You can also tape-off any edges to protect them and to make them clearly visible while you're sanding.
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