Grinding Metal a primer

How to Grind Metal


This information is provided here as a re-post from this article on the 67-72 Chevy Truck Forums.

there seems to be allot of confusion on how to grind metal, I am gonna try my best at helping everyone out some, I still have more to add, but this will be a start. 



6″ Dynabrade for my finish work, this does finer swirl marks in the metal, creating a more uniform appearance. 
6″ Dual Action sander, AKA,”THE KNUCKLE BUSTER” if you know the meaning of this, you have used one. used for medium sanding
8″ Mac sander/grinder, used for initial knockdown and leveling.

“Don’t be afraid to dive in and tackle these tough jobs, the right tools and techniques will see you through!.”


:Grinders, there are several, this is what I use

Bosch 4.5″ electic grinder with either a flap disc or grinding stone, or cut off wheel. This is my favorite.

El Cheapo air grinder for all of my Carbide bits, these are used for corners and any place I can not grind with a stone

Air tool for my cut-off discs and 3″ grinding stones

Mac 90 degree grinder for my ROLOC discs, ranging from 36 to 80 grit, also comes with abrasive pads and surface pads.



:Safety Equipment, A MUST HAVE!!!!!!

proper gloves, I use tig welding gloves
face shield
ear muffs
3M particle mask
Safety googles


So you have a spot weld you want to grind down, what now? Should I just hit it with a hard stone until it’s beyond flat and now I’ve

heated the metal along the way, NOPE. stop right there.

you never want to heat the metal when grinding, especially when it starts turning blue, bad, very bad, you just warped your metal and

now have more work to deal with.

The first thing to do is grab a 3″ hard grinding stone, or one on the electric grinder and slowly, I mean slowly, grid down that weld, get

it close to the surface of the original metal, without touching the original metal. It takes practice, I know. 

Once you have it close to the bottom, switch over to your ROLOC disc and start smoothing it out, never put too much pressure on the

metal itself, just let the grinder do the work for you, nice and easy does it. This will take some getting used to but once it is mastered,

you will have won half the battle.

Now you have some marks either in 36, 50, or 80 grit leftover from the grinder, what now? Leave them? nope, depending on how bad

you have cleaned up that weld, you can now use your sanders and smooth them out.

I always use 80 grit on my pads to do this. Sand it for a bit and you are done, 8 out of 10 ten times, no “mudwork” is needed.

You have just mastered spot weld grinding.



This is where it gets tricky. Lets say you have a weld 10″ long, well that is a major heat source waiting to warp. Time must be taken to

do these types of welds, Patience will prevail.

Well which tool should i start with? For a weld this large I either start with my 3″ air grinder or my Bosch 4.5″ hard stone grinder.

Depends on what you have and how you feel.  Now remember to go slow and let the stone do the work for you, nice and easy,

Do not to rush into things! otherwise it just gets messy.

Grind a small section at a time or go back and forth over it slowly. Never sit in one spot too long or you just heated the metal and

caused more work for yourself. 

Since you have now ground down the majority with the stone, I switch over to my Bosch grinder with a flap disc using 36 grit, this

allows me to remove material quickly without pressure and does a nice smooth job. I never go to hard with this one either.

Start at one end and go back and forth over the metal, these flap discs do an amazing job on leveling the weld and creating a nice

uniform surface. If you still feel wearily about getting too close with an electric grinder, then just finish it up with you ROLOC disc and

smooth it out.

Now that I have gone this far, I will normally take my 8″ MAC sander and smooth over the whole surface with 120 grit. After this is

done, I use my D/A sanders and finish the job, nicely ground down and sanded, nothing looks better than this.



Wire wheels on the electric grinder work great fora variety of cleanup chores such as , seam sealer, old grease and grime, gutters…..
I also have a Hard grinding disc for my MAC 8″ air grinder and this works great for many many welds that need smoothing, pinch weld

seams, stitch welds, frames……

The carbide bits are a god send in my work, you can use them for just about anything, from cleaning up a weld in a hard to reach

area to smoothing out that corner that nothing else can get to, to creating a nice radius in a transition area.

Keep all of you old 3″ air grinder hard discs around, you never know when that one that is almost worn out to nothing will be used

next, but it could be the right diameter for a job that you are trying to get a smooth grind in or transition.

Even worn out ROLOCS come in handy, when they wear out, they tend to bend,well this is perfect for when you are trying to blend in

a radius or doing a corner, they hardly bite and more pressure can be put on them.

Flap discs for the electric grinders have their purposes also, the new ones are great for tight 90’s, and much more, but sometimes

that old used one works better for edges> You can just trim it down some on the edge and you have a brand new, but smaller flap disc!

I hope this helps some people out in the hobby world, there is so much more yet to learn, but this is a start.