This is the procedure I follow to get a car body ready for primer. Everyone has his own procedure to prepare the body, and I have tried several different methods myself, but over time I have adapted this one which works best for me.
The first thing I do once I take the body out of the box, is to inspect it for all possible defects. These include mold marks, sink marks or any damage the body may have incurred during transit. At this stage, I also test fit any parts that need to be attached to the body like bumpers, side-skirts, boot spoiler.. etc. If any part does not fit properly, this is the time to make the necessary alterations. This is known to be a problem with older kits, so it's important to mock up all the body parts and test their exact fit
Next, I see what alterations need to be done. If you're adding photo-etch parts or metal stickers, some molded badges may need to be replaced with these new ones, and this is the time to make the necessary modifications. For example, the Mitsubishi badge on the front grill of the Evolution VII above was going to be replaced with a photo-etched one, so this was cut out and the pillars holding it smoothed out. You may have some badges on the tailgate that need sanding off too. It's important to study the photo-etched set instructions sheet well to see what exact modifications are necessary. If you're planning on doing some custom work on the body like adding putty, styrene or a custom body kit, it's better to do this work at this stage too
Then, I like to take care of any vents that are molded as one piece. These may include vents in the front mudguards or both bumpers, like the small ones above inside the Evolution VII front bumper (right vent was already cut out for comparison). These are usually a fake grill which you need to paint black at a later stage, but they are always unrealistic like that. So I prefer to cut them out and use a spare mesh instead, usually an original Tamiya mesh which is supplied in most of their kits
This is my method to open a vent. I first find a drill bit that is the same diameter as the height of the vent, like the one above
Then I drill a hole on each side of the vent. Take care that the holes are drilled in line, preferably a bit better than mine!!
Then I take a small blade and very carefully cut from one hole to the other until the middle section is separated. Next, I take a small metal file and smooth the edges of the cut. If the vent is not linear, like for example the vents found in the sides of the front bumper on certain sports cars, I still use the same procedure. Instead of just the two edges, I drill a hole on every edge, and then with the blade I join all the holes together until the middle section is cut out. I have seen other modellers drill a hole in the middle of the vent and then sand off from the centre to the edges. It works great too, and you should use the one you work most comfortably with
Then it's time to start taking care of the mold marks. This Fujimi Evolution was full of them, so I couldn't have picked up a better example! Marked above are the typical mold marks you may find. These need to be carefully removed as once you start spraying, these will pop out and make your overall finish look bad. Newer kits have much less mold marks to take care of, however older kits usually have a considerable amount of them, like this Evolution. These are usually situated on the sides of both front and rear bumpers and sometimes along the roof lines. They may be present on the edge of the hood or tailgate too
To remove mold marks, I use a 440-grit grey sandpaper I buy from a local hardware store. It's important not to press the sandpaper hard on the body, just apply enough pressure to feel the sandpaper lightly sanding down the surface. When you notice that the mark is gone, remove any sanding residue and check if it's completely gone. In the picture above you can see the original mold marks on the left side compared with the smoothed surface on the right side
I told you that this Fujimi Evolution was full of mold marks, and here's another proof! The whole roof line, hood and luggage boot sides were also areas that needed attention. Above you can see a comparison of the original roof line (right side), compared with the smoothed one (left side). You can see a considerable difference since the mold marks had a certain height
Here's a close-up picture so that you can see better the difference between the smoothed (left) and original (right) roof lines
One important thing to remember when removing mold lines is to always use a small piece of sand paper. It will be more handy to use, and a big piece of sand paper could result in sanding unnecessary areas without realizing about this
Now, this is something I do but haven't yet met another modeller that actually does it! When I find small pieces protruding from the body (like these windscreen washers) I like to cut these out, drill a tiny hole to mark their exact location, and finally smooth them out. The reason behind this is because of the polishing process. When removing orange peel, the area around these protruding items is difficult to sand properly, and sometimes you either end up with orange peel still present there, or filled with lots of tiny scratches in all directions. So I found it better to remove them and replace them with new styrene pieces later on
The procedure is to first cut out carefully the protruding item (the picture before). Special attention needs to be taken not to damage the hood or surrounding area. Then I take a small drill bit, usually a 0.3mm one, and drill a hole where the washer used to be. This is done so that later on I will know exactly where the original windscreen washers once were, and when I make the new ones out of styrene, I glue them above the holes. For this Evolution, I also removed the key lock on the right side of the trunk (9th picture above). This would also have made polishing around it difficult, so it will be replaced later on too
Finally, I take the same sand paper as before and slowly smooth out all the residue left on the part after cutting out the pieces. This will leave you with something similar to the above picture, with just 2 tiny holes instead of what used to be the washers!
When working with a scriber, it's important not to press the tool hard on the body as this will probably make the line wider, not deeper. Let the weight of the tool scribe while gently moving it along the line. The scriber above is made by Crazy Modeler
This is the other scriber I use, also by Crazy Modeler, and mostly use it for straight panel lines. In the picture above, you can see the scribed material still on the panel line. This is the amount of material I usually remove; just making it a little deeper
Once the panel line is scribed, I take the used sand paper and lightly sand the panel line area. This is done to remove any pieces of plastic that might have been left behind by the scriber. Then, I take the toothbrush and clean the panel line from any dust sitting inside the line. At this stage, I also check the panel line for any damage that might have occured during scribing
This a typical round panel line, a fuel filler cap. For these, I use the other scriber, the pen-like type. Procedure is the same, let the weight of the tool do its job until you see the material coming out of the line. Above picture shows when I stopped scribing
Once again, the next step was to lightly sand down the area around the panel line, clean using the toothbrush and check for any damage. If for any reason the scriber slips and lightly damages the panel line, I make a drop of super glue on the damaged area, wait for it to fully dry, then slowly sand it down and again re-scribe the part. Have always corrected the damage this way!
Now, back to the shiny body! To remove this, I simply wet sand the body and all of its parts using an automotive 1200-grit sandpaper under soapy water. Nothing fancy here, just slowly sand every part, watching out for those sharp edges once again!
Keep a tissue paper handy and from time to time dry the area you're sanding to see if every part has been properly sanded. Once you're sure that everywhere was well sanded, take out the toothbrush and slowly wash the body with clean soapy water
Then I gave the body a good wash with clean water without any soap and here's the result. You can see that the shine is gone
Next step, I blow all the water off the body using an airbrush with the compressor set to 3 bar. You can leave the body to dry by itself, however I prefer this method so that once fully dry I immediately apply a quick layer of primer, as explained below. At this stage, be very careful with the airbrush while drying. You don't want the airbrush tip to hit your freshly smoothed body, especially if your airbrush has a crown tip. That could easilly damage the body and send you filling and sanding the body again
Finally, I apply a quick layer of Mr.Hobby fine primer to highlight any defects that may still be present. The primer will show you any type of damage that needs correction, even if it's a small panel line scratch, and that's the reason I always make this step
And this is the body together with all of it's parts ready for the final coats of primer. For this I use automotive 2K primer as shown in my recent builds. This type of primer requires mixing with hardener and thinner and takes some time to prepare, that's why I prefer performing the step before to be sure that no defects are still present on the body. I don't want to waste time and paint mixing a jar of 2K primer, then realizing I have a scratch and have to stop immediately to correct the damage