PLUG THE PORES What you use to prep the body for paint depends on the chosen finish that you will go with. For a solid color finish you will want to fill any of the pores with a wood filler or Bondo glazing putty. I prefer Bondo because it dries quickly and sands smooth. Use one of those plastic speaders that you can get for mud at a paint or hardware store and press the filler firmly into the pores and gaps in the wood. Cut diagonaly accross and against the grain to fill the pores and gaps better. Use a sanding block and a 220 grit paper and after the filler dries to ensure an even flat surface. Only use your hands to lightly sand on the rounded edges or hard to reah areas of the guitar. The roundness of your fingertips can cause depressions in the woods surface so stick with the sanding block on the flat areas. Inspect the surface to see if any pores or gaps remain and repeat the steps if needed. Then clean the surface with a tack cloth to remove any dust.
Stewart MacDonnald has a great finishing schedule that I would recomend reading before you start the painting process. You shouldn't need to fill any pores on the neck because necks are usualy made from maple which is a tight grain wood. All that's need for it is a sanding with 220 grit paper unless you want to leave the neck natural and unfinished. I recomend using at least a few coats of sanding sealer of clear gloss laquer to protect the wood fromdirt and grime that comes from playing.
thats certainly true if you are using a plunge router but if you don't have access to one and have to use chisels and do it the old fashioned way (like i'll prolly have to lolz), using plywood is nearly impossible, because of the crossing of the grain from ply to ply)
If you want an easier wood to work with, then get basswood. Or, if you like punishment, get a real hardwood like Oak...
But, don't use plywood (you were joking...weren't you?)
with a wood like basswood, you can paint - oil - stain the wood to suit your taste. You can't do all that with plywood.