You need to go much farther into sanding grits.
Lots of wet and dry minimum of of 600g but when I polish I usually got to 1200g and I use water and a drop of dish soap in it but rinse off the soap regularly.
Lots of sanding.
If you get full sheets of SP, cut the accross the center so that you have two 5-1/2 x 8-1/2 pieces and the fold those in 3rds. You end up with a 3 layer thick 5-1/2" x roughly 3" piece of SP.
Let it soak for a few seconds in the tub of water with a couple drops of soap in it to slick it up. try to use as even of pressure as you can for flat sanding. Do not use your fingers, it will make it bumpy and show in the polied finish.
You can use a piece of plastic or hard rubber with a slight rounded edge to sand in between the fins or ridges. Patience is a virtue on polish prep. Better prep better finish, like paint work. Prep, prep, prep then when you are done prep a bit more.
it is easier to ask for forgiveness than permission