ok, there seems to be a little bit of confusion about painting here. I am not gonna try and say that the projects you guys are doing are not turning out good (I have never seen any of them). What I will say, is that some of the things you guys are doing are not correct or condoned by the paint manufacturers, and can shorten the life of the paint job or cause problems.
first i will start with fillers and sanding:
most of your filler sanding can be done with 36 or 40 grit. there is no need to go higher than 80 grit. Most of the filler or sanding primers will fill in the sanding scratches. Body fillers are used to smooth out low spots and make things as "flat" as possible. By "flat" i mean no waves in the piece. As a rule use the filler approperiate for the situation;
long hair fiberglass filler- 1/2-1/4 inch deep
short hair fiberglass filler- 1/4-1/8 inch deep
body filler- 1/8 or less
spot filler(2K glazing putty)- pin holes or very minor imperfections
after you are satisfied with the way it looks and are sure you got out all the major flaws, it is time for the Polyester primer. It takes MEKP as the hardener just like resin does. some brands sand easier than others. I use Featherfill G2 by Evercoat. A few of the other brands i have used with good results are Omni MX245, Duratec, and Sherwin Williams 21. Apply 2-3 coats over areas with filler and 2-3 coats over the entire project for a total of 4-6 coats using a 2.0mm tip. After the poly primer cures (1-3 hours), spray a guide coat. This can be any contrasting color. some people use a light coat of black spray paint, but 3M and others sell guide coat in a can. sand it with 180 grit. USE A SANDING BLOCK or a DA. Never sand without something between your fingers and the sandpaper. It can and most of the time will leave little low spots from your finger tips and/ or palm. Sand the poly primer to 400-600 grit making sure to get rid of all the sanding scratches from the previous grit. If you break through the poly primer dont worry. Once you are at 400-600 grit, of there are large area where the filler is showing, spray a couple more light coats of primer on those spots. If there are just a couple small patches, don't worry about it. If there are any imperfections such as cracks, deep scratches, pin holes, fabric showing through or anything at all, FIX IT NOW. it is your last chance. If all is good, mix up a 2K sealer primer. this give an even color and surface for the base coat (color) to adhere to. sealer should not be sanded unless it is to remove dirt or other things in the primer. if you need to sand it, look at the can to make sure you do not need apply another coat. all you are looking for is even coverage, not thick filling coats like the other primers. most sealers need to sit for at least 1-2 hours before the color can be applied. if left longer than 24 hours, they need to be scuffed and another coat needs to be applied for maximum adhesion.
Base (Color) coat:
get the color code from the car and have the paint mixed to match. Because of the fade some cars have it is wise to shoot a test piece to make sure the paint match is pretty close. if not it can be altered(tinted) at the paint store. Using a 1.3 -1.5mm tip in your gun spray thin coats. depending on the sealer used it may take a few to get an even color over the entire piece(s). usually it takes 2-3 coats for even coverage waithin at least 10 minutes between coats. If there is any dirt in the base color, wet sand with 600-800 grit then reapply the base. This is recommended by ALL the paint manufacturers!!!! once an even color is achieved, let it sit for at least 1 hour before applying the clear. mose base colors can sit for a few days with out top coating. if they sit longer than recommended, there might be an issue with the clear not sticking to the color.
This is the step that brings out the shine and true color of the base coat. It is also will also show any imperfections that you may not have caught before. at this point if you have a pinhole you missed or sanding scratches, you are stuck with them unless you want so sand back through all of the clear and base to fix them. Use the a slower reducer to help the clear flatten or lay down. the downside to using a slower reducer is that the flash time is a little longer between coats and has a little more time to collect dust and other stuff in the air. but the benifit is there is less orange peel.
Spray the clear coat with a 1.2-1.5mm tip (i use a 1.3mm). It works good to apply a lighter layer as a tack coat let it sit for about 10 minutes then shoot on 2-3 medium-heavy wet coats waiting about 10-15 minutes in between coats. when the clear is ready for the next coat, you should be able to touch it and not have it stringy when you remove your finger. make sure you touch an area that will NOT be seen. once you have applied all the coats of clear let it sit over night (12 hours).
sand with 1500 or 2000 grit using a DA or sanding block and sand paper. make sure the entire surface is dull (no shiny spots). using a buffing machine, foam pad, and compound, buff it out. start with a rubbing compound, then machine glaze. use a different foam pad for each different compound. dont turn the machine up too fast or you will burn throught the clear (1500-1800 rpms).
I hope that cleared up some things and will get you practicing with a more accurate picture of the process in your mind.