This started when I took a co-worker to lunch and he accidentally opened the passenger side door onto a tree shrub. The shrub was just coarse enough to scratch an 8 inch long gash on the clearcoat along the lower passenger door panel below the door molding. The gash did not penetrate to the basecoat of the paint, as I could still see the scratched are was still shiny, just that the clearcoat surface was not smooth, slightly "gashed in". The gash was about .25cm thick, and traditional polishing couldn't buff the gash out.

Since I found this gash really ugly and annoying, I thought what the heck, let me try "wet sanding" the scratch out since if I mess up the paint, it's going to the body shop anyway. With some time and patience, I was able to wetsand the scratch from any discerning eye. I will describe my experience and share my technique.

This was performed on a
*2000 Audi A4 Santorin Blue
*Front Passenger Side Door, lower panel below the molding.


DISCLAIMER: 

THE FOLLOWING FAQ, UNLIKE OTHER FAQS, IS EXTREMELY RISKY TO YOUR BELOVED AUDI. IT IS MAY PERMANENTLY RUIN YOUR CAR'S FINISH THAT WOULD REQUIRE REPAINTING YOUR VEHICLE. 

SEVERAL FACTORS COULD CONTRIBUTE TO RUINING YOUR CAR'S FINISH INCLUDING BUT NOT LIMITED TO: INACCURACIES WITHIN THIS TECH ARTICLE, YOUR FAILURE TO PERFORM STEPS DESCRIBED HERE, YOUR OWN CARELESSNESS, THE QUALITY OF THE MATERIALS USED, YOUR EXPERIENCE WITH PAINT (OR LACK THEREOF), YOUR CAR'S CURRENT PAINT CONDITION, THE SEVERITY OF THE SCRATCH(ES). FURTHERMORE, THE LONG TERM EFFECT ON YOUR PAINT FINISH FROM PERFORMING THIS TECH ARTICLE IS UNKNOWN.

I DO NOT CLAIM TO BE A PAINT PROFESSIONAL, THIS ARTICLE DESCRIBES MY EXPERIENCE, ON MY CAR'S PARTICULAR COLOR, ON MY CAR'S CURRENT PAINT CONDITION, ON MY PARTICULAR SCRATCH. 

YOUR RESULTS MAY VARY. USE THIS TECH ARTICLE AT YOUR OWN RISK. IF IN DOUBT, GET YOUR LOCAL BODY SHOP TO DO THE WORK. 

BY READING FURTHER, YOU ASSUME ALL RESPONSIBILITY FOR ANY DIRE RESULTS TO YOUR CAR'S FINISH.

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Please read this ENTIRE article BEFORE doing anything. You will be sorry if you don't.

If you are leery about doing this on your car, find a beater car to practice on (your neighbor's) or go to a junkyard, purchase a test panel, scratch it accordingly, and practice until you have a feel for it. Try to do this on a dark colored panel (black or dark blue, as you can easily tell if you are doing things correctly or not.)

This technique was done on a Santorin Blue Audi A4. I do not know the effectiveness of this technique on any other color. The scratch I was trying to remove was a small 8 inch long, 0.25cm wide gash on the clearcoat, but the scratch did not penetrate to the basecoat. This technique will only work if the gash is on the clearcoat surface only. If you see bare metal, or scratch is too deep, sorry you are out of luck.
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Background: What is wet sanding? In very simple terms, you use sandpaper, and sand your cars clearcoat finish, while keeping the area you are sanding "wet" with some type of lubricant in order to reduce the friction between the sandpaper and your clearcoat. Yes, you heard me right, you are going to sand the clearcoat of your paint.

Huh? Doesn't this make your paint look worse? Well, when you wet sand a clearcoat scratch, you are removing the surrounding unscratched areas to the same level as the scratch. Then, you successively apply liquids and gradually polish the sanded area to a shiny surface.


Materials I Used.

1) Mother's CALIFORNIA GOLD SHOWTIME INSTANT DETAILER (the lubricant : http://www.mothers.com/products/products.html)
2) AT LEAST 1000 grit wet sand paper. The finer grit you can find, the better. Others suggested 1500/2000 grade, but I was too cheap to buy that. MAKE SURE THE SANDPAPER IS WET SANDPAPER (it is black, and waterproof, NOT the beige/orange kind). 
3) 3M PerfectIt II Rubbing Compound. (http://www.3m.com/us/auto_marine_aero/aad/solutions/ent_stain.jhtml)
4) 3M PerfectIt Swirlmark Remover Part#39009 (http://www.3m.com/us/auto_marine_aero/aad/solutions/ent_stain.jhtml)
5) 3M Imperial Hand Glaze (http://www.3m.com/us/auto_marine_aero/aad/solutions/ent_stain.jhtml)
6) Meguiar's Deep Crystal Polish (http://www.meguiars.com)
7) Your favorite wax.
8) Clear tape and plastic wrap (for masking off the area to be sanded) OR masking paint masking material.
9) Lots of cotton polishing rags (make sure rags are for polishing). At least 7 rags needed, one for each of the steps 1-7. NEVER use the same rag for more than one step.

PROCEDURE

Step 0: 

Wash your car, or at least the panel you will be working on. 

Step 1:

Mask off the scratched area. Make sure you leave some surrounding, unscratched paint, because you don't want to just exclusively sand the scratch, otherwise your paint will look funny when you are done. 

For my scratch, I left about 1/2 inch border around the scratch. I just used clear tape with some clear wrap to mask around the scratch. 

Make sure you use light-tack tape, that isn't too sticky, otherwise you'll regret it later.
Also make sure you crease the edges of the tape so you can easily peel it after you are done. I didn't, and it was a PITA to peel the tape off the door (duh!). 

Step 2: 

Take the Mother's CALIFORNIA GOLD SHOWTIME INSTANT DETAILER (material #1).
Spray it liberally in the scratched area so the scratched area is wet. The detailer liquid will function like a lubricant when you actually sand. Make sure that the area you are sanding is ALWAYS lubricated. NEVER NEVER dry sand paint, you will take too much off too quickly and eat the clearcoat too fast. ALWAYS ALWAYS keep the paint you are sanding well lubricated (IE always heavily wet) with the detailer while you sand. 

Step 3

Take a look at the area you are planning to sand. If the area you are sanding is flat-ish, use a small flat object as a backing plate for your sandpaper. On flat-ish surfaces, it helps even out the pressure on the sandpaper. If you're working on curved areas, use your fingers, because you'll be able to distribute pressure more evenly on a curve, whereas the backing plate will place all the pressure on the high spot on the curved panel. Use your judgment. 

Take the wet sandpaper, and pre-wet it with the Detailer liquid. Lightly sand the scratched area, apply very very little pressure, evenly across the area you are sanding, just enough to lightly scuff the area. Sand back-to-forth in a light manner, avoid circling. AS YOU SAND, KEEP THE AREA WET WITH DETAILER. 

Make sure you are careful, don't accidentally sand the paint outside of the masked area (although your mask should protect the covered area). Also, make sure you sand all of the area inside the mask, not just the scratch. You are trying to blend the scratch in to the unscratched border area you masked off, by leveling the surrounding (unscratched) area.

Sand only a few strokes, and then stop. Take a dry rag and dry the area off. I make about 10-12 backward and forward passes, and then dried the area off with a cotton polishing rag. 

Step 4 

Look at the results to see if the scratches are flat with the rest of the paint. It will be slightly difficult hard to tell, because your paint will be scuffed, but do the best you can. If in doubt, don't sand anymore and continue with the next steps. If in the end you're not happy with the results, you can always repeat the process of wet sanding, starting with step 2. IT IS VERY IMPORTANT TO BE PATIENT. Don't try to sand too much at once, you will regret it if you remove all the clearcoat. 

Step 5: 

Take the polishing compound, and a NEW rag and apply to the sanded area. Apply medium pressure, and buff in a backward/forward motion. No circles. Buff the area with a little pressure. The scuff should be slightly smoother as you buff. 

BE CAREFUL WITH POLISHING COMPOUND. It is like liquid sandpaper. If you buff too hard with it and too long, you will eat your clearcoat, just like if you sanded too much with sandpaper. Just be PATIENT and buff with some light pressure, until the paint becomes slightly clearer. 

Polishing compound (despite it's name), won't make your paint shine like new. It's purpose it to smooth out the clearcoat that was scuffed by the wet-sanding. It should make the paint smoother, and the clearcoat slightly clearer, although there will be still some medium/fine polishing scratch marks. 

Step 6: 

After some buffing with the rubbing compound, look at the paint to see if you happy with the results. If you still see the scratch, you need to make a decision. Depending on the depth of the scratch, it may be impossible to wet sand it out completely. Don't expect perfection from wet sanding. First of all, you are probably not a professional. Second, the more you sand, the more clearcoat you are taking away. And even if you sand more, depending on the depth of the scratch, you might not be able to completely remove it. For my scratch, honestly, I can still see it if my eyes are 1 inch from the actual area AND if I knew where to look. So, you decide if you want to wet sand again, or continue. If you want to wet sand more, go back to step 2, otherwise proceed. 

Step 7: 

Take the swirl remover and apply it in the area you just sanded WITH A NEW RAG. Do not use the same rag you used for polishing compound, you not be able to achieve a nice shine if you have polishing compound remains in your rag. 

The swirl remover will make the area smoother, as it contains light abrasives. It basically smooth out the clearcoat where the rubbing compound leaves off. Buff with medium pressure. In this step, I used a combination of back-forth motion and circular motion. Paint detailers insist never to buff in a circular motion. I get mixed results if I only polish in back-forth motion, so I usually start with a circular motion and finish polishing back-forth motion. Just do this until the paint becomes shiny. At the end of this step, your paint should shine, with still some small visible polishing marks. 

Step 8:

Remove the mask CAREFULLY. Slowly peel the tape with as little force as possible, as slowly as you can. Don't rip the tape off quickly, you could accidentally peel the paint (doubt it would happen, but you never know.) 

Step 9 

Continue polishing with the swirl remover, blending the masked area with the surround unmasked area. Use a back-forth motion here. 

Step 10: 

Use a NEW RAG, and apply the hand glaze to the area, using a back-forth motion. Apply medium pressure. This removes small polishing scratches left by the swirl remover. MAKE SURE YOU USE A NEW RAG, otherwise you won't achieve a nice/scratchless shine. Polish until you can't see the scratches no more. 

Step 11: 

You might want to wash off the area with car wash to make sure you polished out all the scratches. The wash will remove some of the polish, and expose any polishing marks you failed to buff out. If you still see some polishing marks, try step 10. If that doesn't help, try step 9 again and then step 10.

Step 12: 

Use a NEW RAG, and apply liquid polish to the area, using a back-forth motion. Polish contains no abrasives, so it just makes the area shine. 

Step 13: 

Apply your favorite wax. 

Step 14: 

Pray your 1-2 hour of work was worth it and that you didn't screw up sanding too deep into the paint. 
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If you are leery about doing this on your car, find a beater car to practice on (your neighbor's) or go to a junkyard, purchase a test panel, scratch it accordingly, and practice until you have a feel for it. Try to do this on a dark colored panel (black or dark blue, as you can easily tell if you are doing things correctly or not.)
Good luck.
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Credits: 

Thanks to Raj (RKA ) for the additional suggestions.