As the title says, when restoring a classic car I've been typically taught it was proper to get the entire vehicle sand blasted to see any issues and make any needed repairs them move to paint. However, is sand blasting safe or does it do more damage then good?
Sandblasting your frame is fine. Sandblasting your body or any panel can be debated until the pigs fly home. People have been doing it for decades, and get by.
However, when you sandblast your body/panels, you run the risk of creating more metal/bodywork for you to do in your restoration. This is because the heat from the sandblaster can warp the panels. If you are not able to repair metal the primitive way, then you are looking at adding body fill to compensate for the damage.
That being said, there are other options.
"Dustless", Water Based Blasting
This type of blasting can also be called slurry blasting. The unit holds water, your abrasive of choice, (preferably glass), and a rust inhibitor. You can thoroughly clean any surface, and not have to worry about the metal warping, or rusting. The water cools the metal, and the rust inhibitor will prevent it from flash rusting.
When you are done, you then spray down the vehicle with an attachment such as the Chameleon. The rust inhibitor is inside the canister to prevent flash rusting while you rinse the abrasive off the panel. You can then dry it with a leaf blower.
After all that work, you are ready to paint immediately if there is no welding/bodywork to be done. The cleaning process is done within the blast. This is the only method which is endorsed by the major paint companies like Sherwin Williams and PPG.
Soda blasting will create comparable results to water based blasting, with one minor downfall. The cleaning process. If you do not thoroughly clean the sprayed application, you can welcome the possibility of severe issues in your paint process. Horror stories of high end paint jobs where the paint flakes off is not uncommon.
You can get your metal dipped acid. This can be cost effective or not depending on your location. Some cities may not even offer this alternative. Size is generally the biggest issue you may encounter with this method.
Lastly, you can use a wire brush. It's very time consuming, and physically exhausting. It can be done, and is an age old method that many people take. You will need a high amp grinder.
Sand blasting is perfectly safe for the metal, especially considering the heavier panels used in classic cars. However, it's less safe for the person doing the sandblasting, due to the higher levels of lead paint and bodywork used in old cars. Pure lead was often used to fill dents and body seams in older cars, and you can imagine what breathing a cloud of that in will do to your lungs. There's also a danger of silicosis from the high-silica sand that is often used. So sand blasting is probably safer for the car than it is for you.
Must be a location based thing, but in the UK sand hasn't been used for many years due to the very real dangers of inhaling even small amounts of silica from the bashed up sand.
Although folks still call it 'sandblasting', in reality it's 'media blasting' and the media varies from tiny plastic or glass beads to ground up walnut shells (believe it or not!).
Avoiding damage to panels is down to the skill of the operator, period. A skilled operator will know what media to use, what pressure, the optimum distance from the metalwork and of course the speed of each pass.
And the joy of working on a vehicle that has been properly stripped shouldn't be underestimated. Bliss when compared to working on a rusty paint covered bucket. At least with a stripped vehicle, you know what you're getting into.
Yes, sand blasting is safe if you're safe
Safety is variable. What is safe? Almost anything can be safe. Exploding bombs can be safe, it just depends on the process you implement for your safety and the safety of others.
Is sand blasting safe?
Not if you sand blast with dioxin laced sand, but even then, you can wear a pressure suit and possibly make it safe for yourself, while your doing it. Don't ever take the suite off and don't ever let your kids within a 1/4 mile of the site though, unless they're downwind, then extend it to a couple of miles.
So, yes, it's safe.
Now to answer your question in the correct context
It's safe for the steel. It may put little tiny dimples into the steel if you hold it over a point for an extended period of time but in the end when you hit it with primer you will need to sand.
Sanding the primer prior to paint will take out any tiny imperfections in the steel. If you strip the steel down completely you will see that it already had some imperfections. The primer has a couple of purposes.
flatten things out so you can clean up imperfections
put something on the steel the paint will stick to
Sand blast away, wear eye protection, send a picture of the completed work, have fun!
Be very careful in getting someone to sandblast your car. Frame, engine compartment, door jambs, underneath are all fine as these are thicker reinforced metal. Quarter panels, door panels, roofs, hood, trunk lid and fenders are not. People will tell you that they have done it many times for high quality paint jobs, then make a mess of it. The heat will warp the metal. I know because it happened to me, from a respectable sand blasting company.