As in all things these days, the preparation and painting of cars has undergone a dramatic change over the last decade. Entirely new procedures are used and many old ones are in the process of being left behind.
Even sand paper is changing. Where my shop used to use the standard grits on paper, we now use sanding discs and strips which are Velcro and plastic backed and are far more versatile and longer lived than the old papers. Not only are they reusable in many cases, the grits are also in some cases more closely related to Scotch-Brite than they are to sand.
When you are stripping the paint from a car the last step, that is physically sanding the car, removes the last of the primers and paint that were left behind by the paint stripper or razor blade. Using eighty grit on a machine is one of the best ways to finish the removal of paint and it also prepares the metal surface for the primers and fillers, which are to follow.
Machines vary and you can use either an electric vibrating sander or air powered feather edger. I would advise the amateur to stay away from large grinders with a sanding disc on them because you can make some very large problems for yourself, either by making very deep scratches in the surface of the metal which you have to eliminate later or, more disastrously, you can overwork an area very quickly, heating it up and warping it. Even a small sander can cause enough heat to warp a large panel, so if you are sanding a hood or trunk with a machine, avoid staying in one area too long or you might end up looking for a new panel to replace the one you wrecked.
There are a number of places to be extra cautious when using a machine to sand your car. As mentioned you have to avoid heat buildup but you also have to avoid doing physical damage to pieces which should not be sanded. I'll bet most of you have seen corkscrew-like scratches on chrome or stainless window trim and on glass. This results from a brush with a sanding machine and it leaves an indelible record of a moment's stupidity. Always make sure that you protect glass or stainless trim before you sand near it. If possible remove it. If not, use several layers of duct tape to protect it and even when masked do every thing you can to avoid touching it with the machine.
Once you have finished sanding the paint remainders from the car, continue sanding until the entire car's metal surfaces have an evenly sanded gray metallic appearance. It is important to have an evenly sanded base on which to start your preparation. Un-sanded areas can cause trouble, even with the new etching primers.
I remember a few years ago when P.P.G. came out with primers called DP 40 and DP 90. The reps told every one all about the miraculous molecular bond, which eliminated the need for heavy sanding etc. I know we were told that the primer actually preferred very lightly sanded surfaces as it bonded so well with the molecules of the steel. Well, what a load of bunk. Four years later paint started to lift on several cars painted with the epoxy and we found that the primer was quite simply letting go to reveal an absolutely rust free and unblemished metal surface below. What was amusing, but only in a dark way, was that the paint company blamed it on the users, stating that there was too much build up of material.
Considering that there were three reps in the booth when one car was painted who were learning tips from our painter, and they found everything perfect at the time meant nothing years later when accountability was called into question.
Now this all occurred several years ago, but don't put any more faith in claims of molecular bonding and shortcuts today either. Those epoxy primers performed admirably when the surface they were to be used on was properly prepared with eighty or even one twenty grit abrasive. The same is true of many of the new miracle products. Preparation is the key to success. Shortcuts will inevitably lead to poor results or worse, a beautiful paint job which starts to fall off the car three years later.
Priming the car really is not something you can do for yourself unless you have your very own spray booth and are set up to shoot modern paints. Long gone are the days when primer can or should be shot in your garage, driveway or even in the shop section of a body shop. I am sure there are still those that shoot primer in an exposed area, but it is a dangerous and stupid thing to do. There are enough toxins in modern paints and primers to drop a carthorse and to expose yourself or any one else to the effects and chemicals contained in them is well beyond foolhardy. A thoroughly modern spray booth is the only place for them to be used, with correct and adequate protection for the user. If etching primers can etch steel, imagine what they can do to your lungs, skin and eyes, the family pets and the kids next door.
From bare metal the next step is your etching primer. Different companies have different colours but what it all does is to seal the metal from rusting and to give a good surface for fillers, high build and colour primers to adhere to.
Any body fill should be applied with primer beneath. It doesn't matter if you break through to bare metal when sanding or working the fill, as there will be lots more primer applied. The initial preparation with eighty grit will allow a mechanical bond with the filler as well so you are well protected from failure.
To all of you lead enthusiasts. Lead was used as filler before anyone invented plastic. It was dangerous, cumbersome to use and had some very bad habits, not the least of which is that it has a tendency to be afflicted with something called lead boil which is oxidization and which causes large bulges, blisters and cracks in your paint. If you have lead repairs in a car it is best to heat and remove them and remember to only do this in a very well ventilated area and use skin and breathing protection. A dust mask does not do it. We have all had far too much chance to fill up with lead poisons in our lives without dramatically adding to the problem ourselves. You can leave original factory lead lines in place but make very sure you use a good sealer over them, as they will not get on well with modern paint.
Applying body filler to a car is not a crime. There is nothing wrong with BONDO if it is used properly. It should not be used to fill holes, dents or deep gauges and seams. That is what metal work is supposed to accomplish.
To say you want no plastic filler in a car is silly, the paint is plastic as is an awful lot of the product that you will use. To have improperly used plastic filler in a car, that is the problem. Too thick and it will crack and fall out and an inexpert hand will create rolling hills and valleys which show beautifully once the paint is on the car.
When using fillers a lot of patience will need to be used and you may have to call in help. Not every one can do it, even given lots of time, but for those willing to try, I'll run you through the methods, dos and don'ts next time.