how do you make a knife blade with scratches look new?

[revencer]

I dont have a dremel

[muskrat man]

a dremel wouldn't be the right tool for the job anyway, I would suggest sanding it down with finer and finer paper untill getting to about 20o0 grit, then polished with your favorite metal polish

[roberts661]

That would depend on the original finish of the blade.
As ground on wheels,polished,bead-blast or coated.
A Dremel saves time for a few scratches to blend them in to the blade material and then finish with the right grit abrasive wheel in a hand drill.It's slow enough that you wont heat up the blade or grab it.
Finish a polished blade with a buffing wheel and the correct compound,then resharpen.
Be extra cautious with older blades that have names or trademarks on them as they are more important than the scratch.
In the Dremel Accessories at the tool store are some german import abrasive buffs(511E) that work well with lower speeds.They come two in the pack for about $5.
My big buff is 4"x1 1/2" on a 1/4" shank with about a 400 grit.

[orvet]

The original finish dictates what I would do to the blade.

If it were polished, I would sand it by hand usually, as MM says to 2000 grit perhaps finer, and then buff it on my buffer.

If the blade had a brushed finished, I would sand it to take out the deep scratches, then finish it with a Scotch Brite wheel or belt.

Just my $0.02,
Dale