Best paint & rust removal tool?

[292farmer]

i have a 4 1/2in electric grinder 12,000rpm and im wondering what is better for removing paint and rust a wire brush attachment or sand paper attachments? i started my tailgate and i went threw 5 sheets of 40grit in 20mins and only did a10in sq section is that normal? sandpaper was from walmart... should i rough up with wire brush then sand paper then wire brush stubborn rust again? is that normal time frame? implanning on doing frame this winter, and started tailgate to familiarize my self with the tool, and find most effective way. thanks