I wet sanded a table top with 400 grit paper and then sprayed a coat of lacquer. I now see swirl marks. Don't remember seeing this when I dry sanded with 400 grit paper. Is wet sanding more aggressive than dry sanding and if I wet sand to I need to go to 600 grit?

 

Not really more aggressive, but the liquid (water or mineral spirits, whatever you are using) helps prevent the sandpaper from loading up, so it cuts better longer. Were you using a power sander? Final sanding before finishing is usually best done by hand and with care to only sand with the grain. Swirl marks are usually a mark of power sanding or sanding in a circular motion rather than in straight lines with the grain.

Also, we are seeing a lot more sandpaper sold in USA that uses the European grading system, which is somewhat different than the US system, especially at higher grits. It is supposed to be marked with a P, as in 400P or P400, but isn't always. You have to go to a higher grit when using P paper to get the same results as with US graded paper. So if you used P400 you would need to go to P600 to get results you are used to with USA graded 400 grit.

[Edit] One more thought...it's important to clean the surface between grits so coarse particles don't remain when you switch to higher grit. Best to wipe the surface down carefully before changing grits.

 

Generally wet sanding isn't necessary when coating wood. Besides what Paul mentioned it is also possible you didn't apply the lacquer thick enough.

 

Thanks for the info on the EU grading system. I was hand sanding, but I am using a water based lacquer which takes 5 days to fully cure. I was sanding after two.