I'm replacing wax on a surfboard with a deckpad. Once the wax is scraped off how should I clean off any remaining wax so that the pad sticks?

after removing the wax, give the surface a good clean with wax and grease remover (aka - prepsol) then a light scuff with some abrasive paper 240-400g

Leave the pad in the sun or a hot car.

Use wet and dry 600g and lightly sand the area.

Clean the area with wax and grease remover.

Stick pad down, leave for 24 hours if you can

Hi,
use turpentine and a rag, bingo!!!

Soften in the sun, scrape with a wax comb then use a rag to wipe and soak wax away. Citrus based cleaner orange power or the like. Prepsol if you're anti environment.

No boiling water and watch the sun exposure.

Use either isopropyl alcohol or meths to get the last bit of the wax off. Thinners or acetone nay stuff yoyr design up depending his they done it, pinstripes etc.

I don't think keying the deck helps much but it must be clean. Keep in mind you will want to take the grip off later.
Once you have stuck your grip on, go around the edges with your thumb pushing it down hard.

Never had issues doing this.
Cheers
Rich