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BMW 3-Series and 4-Series Forum (F30 / F32) | F30POST > Technical Forums > Cosmetic Maintenance: Wash, Wax, Detailing, Repairs > DIY: An Enthusiasts Guide to Colour Sanding
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      09-10-2019, 04:42 AM   #1
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Drives: F32 420d M Sport
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I mentioned in my detailing guide here...


and my spray painting guide here:


...the ability and sometimes necessity to colour sand your car, or it's components. I mentioned that when I next did some, I would provide a guide on how to do it. The context of that comment was in regards to colour sanding your whole car, which is the same principal, but as I wont be doing that until next spring, I figured I would write this up in the context of colour sanding individual components, like M Performance parts that we all either love or hate

The example uses a gloss black single stage painted boot spoiler, but the same principal applies to splitters, skirt extensions, or even your bodywork.

By single stage I mean that the paint is a single stage paint, it painted gloss black in one stage of paint, rather than a base coat and clear coat like on your body panels. Whilst colour sanding clear coat is easier, as you generally have much more paint to work with, the same principals apply whether you are sanding gloss base coat or clear coat.

This guide will be helpful to those that buy the cheaper pre-painted parts and also those that want to paint their own parts. We can't all afford OEM parts and top of the line body shop prices, so its good to be able to do these procedures yourself.

I will also follow up with a guide on how to paint your own parts from scratch. This colour sanding guide would then follow on from the painting stages to complete the process.

So... I am in the process of purchasing M Performance styling parts for my F32 and they will be a combination of ready to paint, second hand, and new but already painted parts. This example uses a very reasonably priced, ready painted, EBay boot spoiler that whilst fits perfectly and is of surprisingly good quality, leaves a lot to be desired on the paint finish.

Some people would be happy with it how it is, but I am a bit OCD and anything I put on my car I want it to look like it has been done by a professional. So my goal here is to correct the issues with the paint only, as the rest is all pretty good.

Issue 1. There are a couple of paint build up spots where the spoiler has been laid on something whilst being painted causing build up around the edges.

Issue 2. There are a couple of paint defects like bubbles in the paint that have burst and then dried like a little crater

Issue 3. The paint is really chunky. We all know what orange peel in paint looks like, and this has it like most things, including your body panels.

As can be seen in the first image below, the paint has a chunky look to it. Look at the area with the reflection of the light bulb in it and you can see the orange peel and how distorted the reflection is.

Section 1 - Sanding
This is the scary part, but it doesn't have to be if you are careful and know what to look out for.

You can perform this with either a grey (finest) Scotchbrite scuff pad, or 2000 grit wet and dry paper. I strongly recommend you go with the wet and dry paper.

Get a container of water with a little washing up detergent in it and a small piece of wet and dry paper. Soak the paper for a few mins in the water.

Either hold the paper flat with your finger tips and thumb or use a small piece of sponge and wrap the paper around that. If you haven't done this before then start off with the sponge until your confidence grows.

Working a small area at a time, like a third of a spoiler, sand in the direction of the longest edge, so down the length of a spoiler, not across it.

Ensure your sanding strokes all go in the same direction.

On a 1 to 10 scale of pressure, you want to be at around a 2 to a 3 only.

BE VERY CAREFUL NEAR ANY EDGES!!! There is less paint on edges and your sanding is magnified as well, so use your finger tips poking proud of the edge of the paper or sponge as a guide to where the edge is and don't sand it!!!

Keep the spoiler and the paper wet at all times, rinsing your paper often so it doesn't get clogged up.

Once you feel like you have covered the section you are working on, dry it down with a towel until it is completely dry to see what things look like. Do this little and often until you are happy with how much effect the sanding is having. You don't want to go at it too hard and go through the paint.

You know you are done when all of the surface you are working on has a smooth, matte "complexion" - What I mean by complexion is, looking at the second image below, you can see where some of the surface is flat and smooth, and some of it is mottled. That mottling is the unevenness in the paint or clear coat. If you polish that back now, the smooth part will be like glass and the mottled part will still show signs of orange peel.

You will likely still have mottled areas close to the edges like in the second picture, so just go over those areas again, carefully until they match the rest of the surface.

Keep going, section by section until the entire piece is flatted back and smooth.

Correct any runs or build ups very carefully. As a non-professional it is up to you how far you take this. Focussing on spot areas like this can easily cause you to go through the paint. Reduce them so they are not noticeable by anyone else and call it done until experience can tell you how much paint you have to work with.

Ignore tips, edges, very angled transitions and small surfaces like the ends of a spoiler tip as you cant really see any orange peel in these areas and they are not only hard to sand without damage, but also hard to polish back.

Section 2 - Stage 1 Polishing
If you have a DA polisher, then great, this won't take long. If not, then you can do this by hand, it just takes longer.

1. Using a DA with a cutting strength spot pad attached, or a coarse hand polishing sponge together with some cutting compound, I like Menzerna Heavy Cut Compound, polish the piece back to a shine.

2. Use a light source to check the reflection. The finish should be glassy, no orange peel and no swirl marks. If you still see orange peel, flat the section back again and re-polish. If you see swirl marks, just polish again for a few more minutes.

Section 3 - Stage 2 Polishing
So assuming your piece is now back to a nice glossy and even finish, repeat the polishing process with a finishing pad or hand sponge and some finishing compound. I like Menzerna SF4800 or 5000.

Your finished piece should now have an absolute mirror finish!

To protect your piece from the elements, before you install it, wax it at the very least, apply your nano coating of choice like Gtechniq C1 at best.

The pictures below show these various stages:

Photo 1. Look at the distortion in the reflection showing the orange peel and chunkiness of the paint

Photo 2. Notice how some of the surface is flatted back nice and evenly but closer to the edges there is still some mottled surface than needs to flatted back further.

Photo 3. Check the reflection of the light source again, you can now see how even it is and now glossy and mirror like the surface is. This is after the stage 1 polish, apologies but I forgot to take a shot of it after the stage 2 polish but it would be hard to demonstrate the difference in a photo anyway.

As mentioned above, you could go on and apply this process to your whole car if you so desired, to entirely remove any defects, swirl marks and orange peel and give your car an absolute mirror like finish. But you have to be really careful of panel lines and edges, so it's best to use a product like Menzerna Liquid Sanding compound and a lambs wool pad to be safe. I'll share my experiences of doing this on my F32 next spring.

Additionally, if you have painted your own parts, you should now follow the above guide to perfect them.

It goes without saying that you follow this guide at your own risk and I take no responsibility for you screwing anything up

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Last edited by B1ue52; 09-16-2019 at 09:05 AM..
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      09-16-2019, 09:14 AM   #2
Private First Class

Drives: F32 420d M Sport
Join Date: Jul 2019
Location: England

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Added a painting with rattle can guide here to demonstrate the painting process before colour sanding:

DIY: An Enthusiasts Guide to Painting with Rattle Cans https://f30.bimmerpost.com/forums/sh....php?t=1653152
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