What Grit Flap Disc For Welding [To Use On Your Metal]

You want that when you’re done welding.

That your weld looks like it’s hardly there.

You’ve seen those finished weld pictures up on social media.

And you want yours to look just like those.

But how to they do that?

Well they of course put in a good looking strong weld.

But the before weld preparation. And after weld clean up. And smoothing is just as important.

And using a flap disc of the right grit size is what you need.

So how do you pick the right grit size for your welding?

Please read on and find out.

What is Grit On A Flap Disc?

Grit is the rough surface on your flap disc.

It’s the part of the flap disc that does all the abrasive work on your welding metal.

And when you go to buy your flap discs, you’ll see there are different grit sizes. So what does this all mean?

The grit size tells you the size of the particles on the surface of your flap disc. You’ll find that the most common sizes are 24, 40, 60, 80 and 120 grit.

You’ll find higher grit sizes like 150 grit and above. And you use them for very fine specialist smoothing work.

You’ll see that not only does grit come in different sizes. Grit comes in different materials. Materials like Aluminum Oxide or Zirconia and or Zirconia-Alumina.

Some flap disc surfaces are better for stainless steel or Aluminum.

So, it’s best you pick not only the grit size for the job. Also pick the surface grit to match the metal you plan to use your flap disc on and weld.

And also know that the lower number grit sizes are rougher and more aggressive on your metal. The high grit sizes are the most gentle. 

How To Pick The Right Grit Size For Your Flap Disc?

Accept that you’re going to need several grit sizes of flap disc when you’re welding?

Why?

You’re going to start with the basic metal. Or parts for weld repair.

And before you start welding, you’ll want to give yourself the best possible chance of a good solid weld. And key to that is clean, smooth metal.

Using the right grit size on your flap disc will get you there.

Start with the grit size that’s the least aggressive on your metal. Yet get you the result you want.

Because you only want to remove what you need to. And leave behind good metal. Aim to leave the most metal you can for your weld.

And that means starting with a higher grit size. Say a 60 grit flap disc.

See if that grit size will do the job for you.

Start lightly.

You use flap discs with a gentle pressure. The idea is the flap disc does the job for you. Not brute strength.

Then you can always move to a rougher grit size for the parts of your welding project that need it.

You can step it up to a 40 grit flap disc if that old weld. Or rust isn’t shifting.

Flap Disc Grit Sizes For Use On Metal For Welding

24 Grit Flap Disc

You’ll find this is a very rough grit size. It’ll remove a lot of metal quickly.

Use on thick metal. Or metal with large burrs you need to smooth before you weld it up. Only use this grit size when you need ultimate aggression on your metal.

40 Grit Flap Disc

You use for getting into and taking out old welds. Because you plan to replace with a brand new weld.

You use a 40 grit flap disc to grind on the edge of your metal. A for instance would be where you’re using your home flux core or MIG welder. And it isn’t up to welding 1/4 inch welds.

You can use a 40 grit flap disc to chamfer or bevel the joint you want to weld. And Prepare the joint for your multiple passes with your home welder.

What about removing mill scale on your thicker mild steel?

Yes, you could use a 40 grit flap disc on that too.

Thinner steel?

Start a bit gentler with a 60 grit flap disc.

Thick galvanized steel to deal with?

Then 40 grit could be what you want.

Use a 40 grit flap disc to grind on your weld for a better look. Especially if you’re a newbie to welding and you’re not happy with your beginner weld.

But you’ll find it harder to get a smooth finish with a 40 grit.

Do you have a light touch?

Then you can get a nice shine with 40 grit on your finished welded project piece.

60 Grit Flap Disc

You pick 60 grit when you’ve rust or paint on your mild steel.

Gentler than a 40 grit flap disc try this first on your thinner metals.

Use to take off dirt on your metal.

You’ll find this is also the grit size to use to deburr thin metal.  And round the edges off your metal for welding.

Or use a 60 grit flap disc for thin galvanization on your steel.

Start with your 60 grit flap disc when you’re not sure if a 40 grit size would be too aggressive to use on your weld project. You can always move to a 40 grit later if your 60 is not quite enough.

And use your 60 grit for finishing off and smoothing the surface of your finished weld.

And to fix those sanding scratches. When you started off with too aggressive a grit for the metal you’re welding.

80 Grit Flap Disc

Is a lighter, softer flap disc. You use 80 grit when you want light grinding. Or nothing too aggressive on your thinner metal. When you’re looking for a more professional finish. Use 80 grit to blend your weld into your surrounding metal.

120 Grit Flap Disc And Above

You pick 120 grit on your flap disc when you want a fine surface finish.

Use this flap disc when you plan to put a lovely finish shine on your weld and metal.

What Grit Size Flap Disc For Mild Steel?

When you’ve thick mild steel 1/4 inch and above. Or heavy rust or jagged edges to deal with then you may want a 24 grit flap disc.

Other than that. 40 and 60 grit are good places to start. Get rid of mill scale and tidy up that joint for welding.

Finish off with 80 Grit or 120 grit for a smooth finish and blend of your weld.

What Grit Size Flap Disc For Stainless Steel?

Do you use a flap disc to grind on stainless steel?

Yes.

Despite it’s shiny look, even new from the factory stainless steel will have surface dirt. Dirt you’ll need to grind off before you weld.

Stainless steel looks tough but it’s easy to damage that shiny look. Pick the grit size according to the end result you want to achieve.

Start with the least aggressive flap disc grit size you can. 80 grit for thinner stainless steel and for a better finish.

Move onto 60 or 40 grit if you’ve existing welds to grind out. Or tough burrs to remove.

But as important as the grit size. You need to pick a grit made with material suitable for using on stainless steel.

You’ll want a flap disc grit surface that runs cool. You don’t want to put scorch mark your stainless steel.

Then pick the grit size you want.

What Grit Size Flap Disc For Aluminum?

A bit like stainless steel. Aluminum looks tough. But has a much lower melting point than mild steel.

So as important as the grit size is. A flap disc designed for Aluminum is what you want. Because Aluminum’s lower melting point will easily clog a flap disc surface that runs too hot.

Zirconia and Zirconia-Alumina are the grit surfaces you want to look for.

Pick a 80 or 60 grit size.

And grind off that Aluminum Oxide.

You’ll find that Aluminum Oxide forms on the surface of your Aluminum within a few minutes. It starts to form as soon as your naked Aluminum is exposed to air.

Always flap disc it off before you weld.

Density Differences In Flap Discs And The Affect On Your Metal

As well as grit sizes. And different surfaces. Flap discs come in different densities. Density in the flap disc world tells you the number of flaps on the disc. And the spacing of those flaps across the surface.

High density jumbo flap discs for example. Have more cloth on the flap discs and are thicker and softer.

Coupled with a 80 or above grit size. They are perfect for finer blending of your weld with your metal.

Standard density flaps are more aggressive on your welding metal. And are better used when you need to work quickly across your metal.

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