This is definitely one of THE most important stages in the entire process.  After the body is completely stripped and hand sanded with 220grit, you are ready to fill in all of the inconsistencies, pits, holes, chips, dings, whatever...  This particular body was basswood so it did not require the use of actual grain filler, just some spot applications of Elmers Wood Fill with a small putty knife.

Some of the more porous woods such as Mahogany will definitely need to be treated with a grain filler, you can find this at WoodCraft or possibly Lowes. 

Before you apply the sanding sealer, you need to raise all of the grain and sand down with 220grit.  To do this, lightly wet a rag and gently brush over the body.  After the water dries, the body will no longer be smooth.  Sand with 220grit and repeat as necessary.  This body took 4 sessions.

Now that you have raised the grain and re-sanded the body, you are ready to apply your sanding sealer.  I used Minwax brush on type that I bought at Home Depot.  You can also buy sanding sealer in an aerosol can, but I prefer the brush on type, and I use the little 50 cent throaway Home Depot foam brushes.

To prepare for the sanding dealer, block sand the body with 120grit sand paper, using even uniform strokes, with the grain.  Repeatedly check your sandpaper for any loading, and continually brush or blow off the dust as it builds up.  When done, spray with a compressor or canned air, and thoroughly whipe the body clean with nahptha.  Now we can brush or spray on our sanding sealer.

Once you are satisfied with how the body looks, and are positive it is flat, (look at it from ALL angles, fix ANY imperfections NOW) you are ready to apply the primer coat....

I want to talk a little more about how important this stage is.  Don't do this just because it's a step to do along the way...  Understand what the sanding sealer is doing, and why it might take a lot of coats, and a lot of sanding until the body is ready.

The sanding sealer will fill grain, and it will also build up low spots in the finish, ie: dips, dings, scratches.  Do one coat, let it dry 3 hours, then do another, coat let it dry overnight.  When you sand the next day, use a LEVEL sanding block and make sure all of your strokes are flat.  Around the cutaways you will have to use sandpaper and a small 8mm or 10mm dowel.  On this first sand after your initial coats have dried, do NOT remove all of the shiny spots.  The shiny spots are the low, pitted, dings, whatever areas and it needs to stay shiny there.  Some of you might freehand sand down into dipped areas, or think you need to remove the shine/sealer there and have it totally flat.  DONT.  You want the shinies to stay.

Sand it flat all around with 220grit. leaving the shiny spots.  (If you were lucky enough to have a perfectly, completely, pristinely flat body with absolutely no imperfections whatseoever, you can get away with 2 coats.  You will know after sanding because there won't be any shiny spots after block sanding).  If your body was like this one, pretty tweaked, put on another coat, let dry three hours, then do another coat and let it dry overnight.  Then sand again.  Slowly you will start to have fewer shiny spots.  If there is a really persistent valley or dip, you can drop fill that one area and let it dry at least 36 hours if it was deep, or overnight if it wasn't too massive.  Don't prime or do anything until you can coat your body with sanding sealer, sand the body flat, and have it be completely matte with no shiny spots.  This means you are dead level.  If you don't, even a small little dip will show up like Rosie O'Donnells ass when you final polish.  We clear?  Good.