Whenever its possible, it gives you a much heavier sound
Dig a tunnel of light through ignorant walls
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Whenever its possible, it gives you a much heavier sound

This. Some of the heaviest songs were done with downstrokes. Kerry King and James Hetfield are two of the baddest mothers when it comes to downstroking. It's important to try to downstroke when you can. This actually improves your sound and tone and technique all in one.
Ideally, your upstrokes should have the same strength as your downstrokes. But yeah, you can handle most metal with primarily downstrokes as long as you don't overlook your alternate picking technique.
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I look at it this way - if it needs to sound really heavy, it needs to be downstroked. But you can only achieve a certain speed by downstroking (at least for any duration). So if you're attempting to play an extremely fast riff that extends throughout a song, you will probably want to alternate pick it. Certain sounds such as galloping are only possible with alternate picking. You'll want to be able to do that to add variety to your metal playing.

So I guess my rule is, if I can downpick it comfortably for a whole song without tiring, then downpicking is okay - but if it's faster, thrashier stuff and beyond, I don't worry about having all downstrokes. And with alt picking you can still emphasize the downstrokes to retain the "heavy" sound.
Atmospheric dark metal w/ black and death metal influences:
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you should be able to downstroke at half the speed you alt pick also
Downpicking does give a heavier sound, or for some, a more even tone.....it would also be good to work on your upstroke so that it delivers the same strength as your downstrokes....

I suggest practicing both methods.....using each whenever you feel like it or whenever necessary OR to achieve a "strong metal attitude".....haha.....see what I did there....yeah I know, I'm awesome.....

"Well, yeah, sometimes I get a little too creative."
~Bruce Dickinson~

"Various equipments"
When YOU want the sound of downstrokes.
Actually called Mark!

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