These are the steps that I take while doing my body filer and then primer right up to paint.
From bare metal I Epoxy Prime everything. Use what ever Name Brand you like. They are pretty much all the same. I like to give my EP a couple days to cure but it's not absolutely necessary.
I then give the panel I'm going to use filler on a good scrub with a red etch pad. This is equivalent to about 320 sand paper, but I find it faster and you don't take off any material. I just ruffs it up a bit.
Filler work.... this is subjective but this is generally how I do it.
Large open panels I start with 40 grit to ruff out the shape being careful not to take of to much material.
Smaller areas I generally start with 80 grit.
I almost never use any sort of air sander or air board. I find I have better control with hand boards and blocks. It takes more work but the results speak for themselves
Then I move to 80 grit. This is still very aggressive but is good for bringing in the final shape.
It's usually at this point that I know weather or not I need to recoat the panel. I try to do it in one skim if I can. If I know I have a particularly deep area to fill such as a seam on a 1/4 panel I will give that area a fill coat first before I do my full skim coat.
I never ever sand my second coat (if needed) with 40 grit. I begin sanding the second coat with 80 grit.
In either case I'm now finished with 80 grit and I'll move on to 120. I'm now bringing my surface very close to it's final shape. Experience has taught me when to leave one grit and move to the next. The idea being to never remove to much material which will require a re coat. this is important because if you have to re coat anything you must do the entire panel again. if you do a small area you will find as you sand down again that your going to take out to much material from the surrounding area of the re coat and end up with a wavy panel.
The last step on the filler work is 220. some guys think. (even professionals) that 80 or 120 is far enough and start priming. The problem you will run into is that these grits leave very deep valleys in the filler that the primer will bridge. Leaving a small gap under the primer surface. This will later come back to haunt you when you paint the car. After about a week or two you will start to see what looks like scratches, but this is actually your paint surface sinking into these unfilled valleys. I should mention to that the same holds true when doing your filler work. Never ever recoat your filler over top of anything less than 80 grit. The same problem will arise. This is why I try to do all my filler in one coat.
Now you can prime. Use a good quality high build primer and you will be much happier with the results.
Most restorations that I do get primed twice. The first priming is used for my final straightening. If you're not that fussy you can easily get away with one prime.
I use a spray on guide coat which will help to highlight any low areas and also help to show what has been sanded and what needs more sanding.
I start with 220 grit again. Not pushing to hard, I let the sandpaper do the work. You may find some areas showing through to the filler again. At this point your finished with 220 and moving on to 320 grit. I again spay on a guide coat, and then sand with 320 until all the 220 sanding scratches are gone.
At this point I re prime the job.
Now again it depends on how fussy you want to be and also what kind of paint and color your using, but this is how most of my jobs get final sanded ready for paint.
After the re prime I again spay on a guide coat and begin sanding with 400 grit dry paper. I sand just until the orange peal surface is gone. I spay guide coat one more time. Now you have a choice here. I do my final sand with either 600 wet or 800 dry. I usually go with the wet as it is quicker and less dusty. Your now ready to throw on a show quality paint job.
However, now the real work starts.......... the color sanding and polishing.
and everybody thinks it's easy.
