which sandpaper do I need to use to scuff the paint?

This is preparing the surface for waterproof painting. any tips would be great. 

Any medium grade sandpaper will do if it's a bigger grade it doesn't matter, you just need to dull the finish and take the sheen off to key the paint so the paint will adhere

What is the surface you are painting on? 

I am not a painter but generally you don't need very coarse sandpaper to get a good adhesion. 240-320 grit maybe? It also depends on how mint the end result should be, if the surface is flat or bumpy, etc. But it's more important to have a clean, oil free surface and lay down some primer.

Again, not a painter, just my experience.

I assume you are referring to Protectakoting your van floor, once sanded you need to wipe the surface down with xylene thinners or similar this stuff sticks to anything have a read through this https://windowcleaningforums.co.uk/forums/topic/26070-protectakote/?tab=comments#comment-378289

Iron Giant said:

I assume you are referring to Protectakoting your van floor, once sanded you need to wipe the surface down with xylene thinners or similar this stuff sticks to anything have a read through this https://windowcleaningforums.co.uk/forums/topic/26070-protectakote/?tab=comments#comment-378289

Click to expand...

it's actually bully liner I'm using. will check out

Just use 3m scotch pads perfect for the job