I'm not familiar with the filler you are using, typically filler would be used prior to finish....
I would not wet stock after you start finishing process, typically the wetting process is done on bare wood... not that you can't wet the finish but, the grain probably won't raise a lot after...
Here is a 870 I refinished... it looked terrible wish I had a picture before I started it was rattle canned o.d. green to start when I bought it at a Pawn shop.
No expert here but, I worked in a custom cabinet shop for close to 10 yrs... The vertical lines are part of the great part of walnut, (desirable) fiddleback, looks like a very good looking hunk of wood you have there! You might sand to 600 or 800 grit before being perfectly smooth but, the marks...
7mm bullets
$40 per box
3 boxes of 100 each Hornady Interlock 139 gr.
1 box of 100 Hornady AMAX 162 gr. SOLD to Justice1327
1 box of 100 Hornady Interlock 162 gr.
3 boxes of 100 each ELD-X 162gr. SOLD to Justice 1327
1 box of 100 Speer BTSP 160 gr.
bullets will be shipped using USPS flat rate...