Sanding Surface For Painting

Butch is entirely correct about what it is propelling your sandpaper. I use three different grits (80,220and400) for the most part. I have some 320 but rarely use it. I like the 3M hook and loop discs used on an electric random orbital (RO) sander. This is similar to Butch's DA (dual action) pneumatic sander. You can actually use a coarser grit with that type of motion that you can with just a circular disc sander. I know 180 or thereabouts is recommended prior to initial priming but I usually use 80 for that purpose prior to epoxy on sheet metal, if it needs sanding at all, unless it is sandblasted. I prime with the epoxy after the 80 grit knowing that it will be followed up with surfacer. Then two or three coats of surfacer. Then machine sand with 220. Then a final coat of surfacer hand sanded (dry) with 400 immediately prior to topcoat. That's the routine that has worked OK for me, despite the fact it is coarser than sometimes recommended.

The other scenario is if you are using sandpaper to actually strip existing paint or to remove rust. For that I would use 24 or 36 grit "paint buster" discs on a circular sander, followed up by 80 or finer on an RO or DA prior to priming.

The most important aspect is the final sand prior to topcoat. NO evidence of sandscratches should be present at that point. The smallest imperfection will show up after the topcoat is applied. A final sand of surfacer with 400-600 is typical for most topcoats that I am aware of.

Rod