What type of sandpaper do I use on a dewalt dw421?

You can get a new one fairly cheap. Toolpartsdirect.com there is different types to that model. Once you get it, they are not hard to change, 3 or 4 screws.

Probably need to replace bearing 6202. I took mine apart, dissambled the retainer and used PB Blaster on the bearing. It seems to be a short term fix. At least it's not operating at hyper speed anymore. Long term, I probably need to replace the bearing and maybe a new lower retainer if I can't press the bearing out.

When using this kind of sander you have to let the machine do the work, meaning if you press on it things like that happen, if you aren't doing that then possibly too thin and cheap sandpaper attached.

I'm not sure I understand your question, but I'll give it a go.

The DW421 is a sander already. If you want to change it to a polisher, you can buy polishing pads to go on it.
If you have a Dewalt polisher and want to convert to a sander, you can buy sanding disks to fit your polisher. They are installed on the spindle and I believe you can get a backing pad for it also.

Hope that helps

What do you expect?
Probably mostly plastic & cheap
See if Home Depot will take back

Impossible not to get water in the bearings and motor. It is vibrating too fast. You need air sander. Or sand dry, then wet sand the primer by hand.

I think your model holds the paper with glue that is on the sand paper.Just take apart the back cover of the sand paper to expose the glue.

Seems time for a new sander.

Do not break anything. Go to Dewaltservicenet.com and type in your model number in the middle of the page at the bottom and download a user manual for the tool. It should have come with what is called a punch plate to perforate the sandpaper for dust pickup (such as it is) Go there first and you will find everything that you need to use the tool

dewalt has a web sit where they answer your questions