Originally Posted by intrepid lover
I am planning on painting the hood of my car. I was told by someone that I needed to wet sand my hood before I paint it. Is this true? Any suggestions on what sort of wet sander i should use?
I'm sure most of you have heard the term "wet sanding" or "color sanding." What do these terms mean and when and how should you use these procedures? Sometimes the only way to remove or lighten a heavy imperfection is by sanding the paint surface. This can be a fast and effective way to remove a heavy scratch or scuff, if you are skilled and very careful. On the other hand, this method may lead to a trip to the spray booth if you are over zealous and not aware of when to stop.
During the process of wet sanding you are actually "shaving" a minute layer of clear coat off the paint surface to level out a deep scratch. This is also the process used to remove scuffs or marks left by other cars. Wet sanding is also used in many body shop procedures such as evening out a run or sag, eliminating dry spray, "knocking down" too much orange peel, or getting rid of dirt that got caught on the finish during the paint process. In many cases wet sanding can be more effective than trying to compound these imperfections out. Wet sanding will not create the heat that compounding will, but many times is more aggressive, and special care must be taken throughout the entire process.
CHOOSING YOUR WEAPONS
Sandpaper comes in a variety of grits. Similar to sanding wood, you start with a more aggressive paper and graduate to a finer paper, which will remove deep sandscratches and produce a smooth surface. The lower the number the more aggressive the paper will be.
Grits will range from 400 to 3000. The most popular and common papers to use are 1500 and 2000 grit. For vehicle applications, a grit no more aggressive than 1000 should be used. If you use a more aggressive grade you must remember not to shave more than .3 to .4 mils off the clearcoat surface or premature failure may result. The UV blockers are in the top .5 mils of clear. If you proceed deeper than that the clear will have no UV protection from the sun. Also paper that is too aggressive will actually rub right through the clear and down to the basecoat. It is an unmistakable sight when you hit that tragic point of "ruining" a panel by sanding too far.
The term "wet sanding" is just that. The panel you are working on is lubricated with water as well as the paper itself. Some brands of paper must be pre- soaked for a specific time period before they may be used. Other companies make paper that just needs a quick dip in a bucket of water before you may begin. You should have a water bucket to soak the papers in, adding just a couple of drops of soap for better lubrication. You will also need a sanding block to back the paper, made of some type of foam or soft rubber, to distribute even pressure across the panel. Don't use your hand or fingers to back the paper because it will create uneven pressure and may lead to sanding through the paint faster. Keep a spray bottle filled with water handy to keep the area wet. A clean dry towel will be needed to wipe off the area to check on your progress.
SAND IT OR BUFF IT?
All imperfections in a paint surface are different and different methods will be used to remove or lighten them. Your skill level, knowledge and expertise will determine if the problem can be fixed or improved without refinishing the area. You would not want to sand something that may be removed by the use of clay or light polishing. On the other hand you would not try to polish out a deep key scratch. Compounding the area may be an option if the scratch is not too deep. A thing to remember is that you will be working in the same area with the buffer until the scratch lightens up or is removed. Be careful about the heat and friction you will be creating and the aggressiveness of both the buffing pad and the compound you choose. Sometimes this method will rub through the paint quicker than wet sanding.
Once you become skilled at wet sanding and are confident that you will not rub through the paint, this method can become a money maker and show customers that your skill and knowledge is superior to your competition. Don't let customers assume that all scratches can be "compounded out." Explain to them that wet sanding and then compounding may be the only option, but it is a risky process and will be more costly.
The color of the car will often determine how bad a scratch appears, and how well it can be repaired. Clear coat will always look whitish in appearance when it is scratched. The two key elements are how deep is it and what color is the vehicle? A scratch on a black car will always look worse than on white or beige because of the background or the basecoat. A scratch on black will have to be sanded deeper to totally remove it. Clearcoat will also leave a "scar" and it may still be visible if viewed from a certain angle. Never try to be a hero and assume that something will be completely eliminated. My opinion is that lightening a scratch is a better option than rubbing through the paint, or painting a stripe on the car with touch up paint. Just remember that if you rub through the paint, you lose unless the customer was willing to have the panel refinished if the sanding process was unsuccessful.
It is always a good idea to soak the papers you will be using even if the manufacturer says it isn't necessary. While the papers are soaking, mask off any areas that are not to be sanded. This is especially important for areas such as bodylines, pinstripes, panel edges, and moldings. You never want to sand more than you have to and risk damage. Wrap the piece of sandpaper around the sanding block and be sure it is free of any dirt or grit. Take a spray bottle filled with water and wet down the area to be sanded. I prefer to sand at a 45-degree angle to the scratch. This method will help you determine if the scratch has been eliminated because you will see the sandscratches at a different angle than the vehicle scratch.
With all the water involved in this process you may lose sight of the scratch and mistakenly think it is removed. The water will fill the scratch and hide it. You will also notice that the water runoff has a whitish color to it. This is the clearcoat being washed away. You may even get a sniff of paint as you are sanding. The key to this procedure is to apply firm but even pressure to the panel and don't stay in one area too long or you will rub through the clearcoat. Periodically wipe the area dry with a clean soft towel and use a blowgun to totally dry the area. At this point you will see no shine whatsoever to the panel, and hopefully no scratch. If the scratch still shows through the sandscratches, it means the paper you used was not aggressive enough, or the scratch is too deep to be removed completely. This is where you must decide to use a more aggressive paper or finish the procedure and hope for the best.
Even if the scratch is gone or lightened up sufficiently, there is still more work to be done. You must follow up with a lighter grade of paper or two to make the buffing process easier. For example, if you started with 1200 grit, follow that with 1500 grit, and then move to 2000 grit. The finer you go with the paper, the easier it is to buff those sandscratches out. You never want to see sandscratches through the clear. This is why I like to finish everything with 2000 paper. It will not take that much more time. At this point don't lose concentration. If you did not rub through the clear with your 1000 or 1200 paper, don't assume that you can't rub through the panel with 1500 or 2000 paper. You are still "sanding" the panel even with the lighter grades of paper.
GET THE SHINE BACK
Congratulations! You sanded out the scratch but look what has happened to the panel. There is no gloss! The finish is white and chalky looking. Now it is time to buff the sandscratches. There are a number of ways to go as far a pad choice (wool or foam) and compound or polish. If you have sanded to 2000 grit, the sandscratches are not that difficult to remove but as stated in previous articles you must be careful not to create too much heat and burn the paint. Don't be in too much of a hurry to see the shine come back. It may not appear on your first or second pass, but it will start to return. Work slowly and gradually proceed to a polishing procedure to remove any swirl marks and bring up a high gloss.
At this point you should have a nice deep gloss with no swirl marks and no sandscratches showing through the clear. Be aware that remnants of the scratch may still appear if it was very deep. Deep scratches in clearcoat leave what looks like a "scar" if they have not been totally eliminated. This is not your fault; it is the nature of a clearcoat surface. Explain to the customer that if you went any deeper with the sanding process you may have ruined the finish, resulting in the need for refinishing of that panel.
Wet sanding can be risky if you don't quite have the grasp of how to do it. Always explain the process to the customer and ask him if he is willing to have the panel refinished if the scratch is not completely removed, or if you rub through the clear. His answer will determine how aggressive you can be. Don?t set yourself up for problems by trying to be a hero. Ask the local body shop for discarded panels that still have a large area of good paint and practice- practice- practice! You may never be perfect but practice will increase your skill and knowledge and improve you ability to make accurate judgements about future repairs.