How to Paint a Car

There are 28 common defects in car painting. Just one defect will ruin all your hard work. The factory invests millions into systems to be able to paint a car only once and have it last for a very long time. To avoid these defects you need to do each step correctly. These are the seven processes involved in painting an automobile:
Choosing the correct system.
Applying bodyfiller
Choosing the correct reducers, hardeners and surfacers for the conditions.
Application of the coatings.
Correct equipment and maintenance.

You need to degrease between all steps in painting.
When degreasing you need to remove all contaminants.
Sometimes you need to use soap and water, sometimes a strong solvent, sometimes a non-flammable degreaser.
There are many different degreasers. Soap and water will remove salt and skin oil but a solvent based degreaser will not touch it at all. If you see the degreaser bead up on the surface then it isn't clean. The surface should allow the degreaser to make a smooth film over it. If not then you need to keep degreasing. Each contaminant can only be dissolved by products of the same family. Acetone won't dissolve salt. Don't touch the surface with your bare hands.

What are you cleaning?
1. Degreasing old finishes and new parts:
Old finishes need to be cleaned with soap and water first.
Use clean water and soap.
Then a degreaser for painted surfaces with clean cloths.
After sanding with the correct grit you need to clean with soap and water again because your hands contain salt and oil which will cause a defect, unless you sanded with gloves or were careful not to touch the surface.
Then a degreaser again. Use clean new cloths. Use one cloth to put it on and one to take it off. Don't let it evaporate on the surface. Make sure you remove it from the surface. Do this until the cloths come up clean. If the cleanup cloth is dirty then you need to degrease again. The surface is ready for the next step.
Degreasing plastic parts.
The only difference is the degreaser used. It must be non-flammable because plastic parts will store a static charge from any sanding or wiping just like a nylon rug and it can catch fire. This has happened four times in the U.S.. Plastic parts absorb solvents so you need to let them air out before you go on to the next step.

Sanding is done for one purpose only, to provide the correct size scratches for the next product to adhere to. Scratching the surface removes some material and increases the surface area. The larger the surface area the better the adhesion. This must be done according to the specs of the next product to be applied. A product like primer may have particles in it that are 3 Mils. in size. If the scratch is too big or too small it won't adhere correctly and will eventually fail. Remember the steel and plastic parts expand and contract wildly with temperature.
It may be 200 degrees on the surface of a car in Las Vegas and 20 below in Maine. If the scratch and the product don't match it will fail. Read the directions very carefully. For example bare aluminum needs to be sanded only with a red scotchbrite but bare steel needs to be sanded with 120 to 180 grit sandpaper for some bodyfillers. Using the wrong size scratch leads to many problems. Many people think that rougher sandpaper means that the surface is better "sanded". Actually if you use an 80 grit piece of sandpaper it means you only put 80 scratches per square inch. So if you used 240 grit you would be adding 200 percent more surface area for adhesion of the next product. Follow the directions exactly. When sanding use a hard backing with the paper if you are trying to level a surface. If you are sanding contours you need to use a flexible but firm backing. Change sandpaper often. If you use sandpaper after it's usable life it will actually be coarser and ruin your paint job by leaving too deep of a scratch.

I can only give general tips on this because each product is so different. Only use fresh bodyfiller. The shelf life is six months. It's too cheap to even take a chance on having it cause a problem later on so buy fresh stuff. Don't overharden it. The ratio is three percent hardener maximum. The hardener is only a catalyst. It generates the heat necessary to allow the two components which are already mixed together to react with each other. Yes, bondo will harden by itself if you apply heat to it without hardener. If you add too much hardener it will eventually get to the topcoat and bleach it a lighter shade. Peroxide is the hardener and also what is used to bleach hair. When applying the bodyfiller make sure the surface is clean and force the first coat into the metal with the spreader as much as you can for good adhesion. Do not use a cheese grater! Let the product harden by itself completely. A cheese grater only works when the filler is still soft so you may actually loosen it from the surface because it doesn't reach full adhesion until it's hard. Also a cheese grater removes the wax that is in the bodyfiller to shield the filler mixture from reacting with oxygen in the air which weakens the bonding of the filler. After it is hard use a wax and grease remover first to remove this coating and your sandpaper will not clog up when you start sanding. Apply the filler from the center out. This tends to make less ridges in the repair.
It's best not to put bodyfiller on paint even though you can do it with some fillers. Don't put more filler on than the recommended thickness. It's on the can. It can be as little as 3/8th's of an inch.

Read the tech sheets for all products or ask somebody who knows.

This is a skill that is learned by doing it. You can take courses from any major paint company at their training centers or find a good painter.

Every single piece of equipment should be in top condition. Clean filters, correctly sized equipment, clean spray guns, clean tack cloths, etc. When you go into a booth to spray a car most of the work has already been done so there should be no surprises of any type if all equipment is maintained according to it's specifications.