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You need to prep the surface to receive paint, here's how.

Prep work

Before you can even think about getting paint on a car or panel you have to prepare it. In many cases you did a rust repair or got rid of a dent, deep scratches or just want to give it a new layer of paint to make it look right. In this case my Mercedes SL convertible has a problem with the clear coat. I'm going to paint the bonnet (hood) to get rid of that horrible paint that's on it right now. The bonnet has some stone chips as well, while I'm at it I will fill those with polyester filler.

So, where do we start? I'm a great fan of taking all parts off the car that are directly on, in or just beside the part(s) to be painted. It's easier to mask off the parts that don't need to be painted and it gives a far more professional result. You won't see where it was masked off!

What I did was take the grill out, removed the badge from the bonnet (it has to be replaced by a new one anyway), took the plastic side skirts off and took the indicators out. Now you're wondering why I took the indicators off, they are not directly in the bonnet. Well, I want to blend the new paint into the existing paint so that íf there is a slight difference in color you won't notice it. Blending is another chapter which I will make when I'm ready to paint. The item is already in the menu but will get a link when that page is ready.

Having taken everything off, the Mercedes looks like this.


As you can see I did not take the headlights out nor the bumper off, since the wings (fenders) are not rusted or anything I will just mask those parts off.

The logo which will be replaced by a brand new one.


The side skirts, unfortunately I had to take the skirts off of the doors to be able to remove the ones on the wings (fenders).



Now I'm ready to start sanding the bonnet and wings.


Very first step to take is to wash the car thoroughly, get al dirt and sand off because when it's still on there it will make scratches you don't want. Do not go to the carwash! Use only water and a clean sponge. Rinse the car and with the water hose on, start to wash it. That way you'll get every small grain of sand off.

I prefer to sand first, before I start filling the stone chips and the 3 dents. The reason is the body filler needs something to stick to, if you apply it to a bright shiny surface your work can be compromised. Wax and contamination of the paint by pollution will be trapped under the filler if you don't sand first.

Before I start I'd like to tell you something about sanding blocks, they come in different forms and are from rock-hard to spongy soft. You want to use a hard block which still has some flexibility in it for the flat area's, on curves you want a soft block. For the fist pass of sanding I like to use this one, it's got a soft and a ridged side.


Extremely hard blocks or even a small piece of wood can be used to get runs out of the clear coat, I'll discuss that in another chapter.

If you're doing bodywork more often and you want to get perfect results every time you may consider buying a set of Durablocks. In my opinion the best you can get. This set was about $30,- ex. shipping, They are firm but not like cork or something. There are different sizes, the bigger the sanding block the more ridged it is. Keep in mind that a car has NO flat surfaces whatsoever, so you want a little bit of flex in your sanding block on panels which look flat.

No, I'm not sponsored by those people, I just want to share what works for me. To be very open about this whole website, I do pay for it myself without getting anything from anybody. No money and most certainly no free products! I do not provide links to sites where you can buy stuff, if you really want it you can find it on the internet like I did.



Moving on to the sandpaper to use, I like to sand panels which have clear coat on them and are only to be roughed up 400 grit wet and dry. If a panel does not need any repairs done to it but has to be sanded to blend the color, I prefer 600 grit wet and dry. With water based paints you want to use the sandpaper dry, all other paints and clear coats you can sand wet.

The fender (wing) has been sanded with 600 grit wet paper, it's still wet from rinsing it but it's silky smooth.


On the bonnet you can see the difference in sandpaper grit, the lighter spots are done with 400 grit wet.


The reason I sanded parts of the bonnet using 400 grit is I had to get rid of that paint, which in it's turn was the reason I wanted to repaint the bonnet to start with. 600 Grit just didn't cut the mustard like they say.


Completely sanded out. The reason I decided for this approach is I don't know what kind of paint it is. If it happens to be just spray can stuff it may react with my 2K basecoat. Yes, there are isolation paints available which can be used as a precaution, so that whatever is under it won't react to the 2K primer I'm using. Using it (I do have it) would involve another step in the painting process, I just decided to put some muscle in it and get rid of it all together.


I do have to do some repairs as well, the 400 grit sanding helps the polyester filler to really hold on to something. Sanding is basically making scratches, you don't want them to deep, just enough to get an adhesion which will prevent it coming loose in the future.

That brings us to the next step, body panel and stone-chip repair.

Repairing body panels.

Yes, this is also a part of the preparation. I'll show the picture first and then I'll explain what's going on.

I will not go into big damages, that will be another subject (yet to come) on this site.


Right, I took some 2 component polyester filler and went over the stone-chips. There were 2 slight dents in the bonnet, one somewhere in the middle and the other one on the side of the grease in the bonnet. I just used a putty knife to apply it. You can buy special spreaders but I don't see the advantage of using those. I'll explain why, You want to put the filler on quite thick so it can be sanded back, just skimming the problem area's is not going to work. You'll cut into the existing paint and you risk sanding through it to the bare metal. It's not always avoidable but an excess of filler gives you some body to work with. In the end most of it will be sanded off.

Let's break it up in 2 different kinds of repairs, let's start with the stone-chips. You can see in the next picture I did not put very much filler on that spot (the rest of the stone-chip repairs look the same. It saves material and is easy to sand back.


Using the first sanding block I showed in this section with 400 grit wet sandpaper gives this result.


It's the white spot at the left, this is how it looks after sanding it.

This is how to get the bonnet perfect before we even think about paint. Be aware, if there is rust in those stone-chips you will have to treat the rust before putting anything that resembles a filler over it. It will still rust after you covered it up and over time it'll come back. Be sure you only made the problem worse if you didn't treat the rust.

Now the dents, they are not very deep but it doesn't look nice. If you're going to paint the whole part you might as well make sure it's perfect!

You shouldn't fill dents which are deeper than like 1/4" or about 6mm, if possible avoid even that. These are about 1/8" or 3mm deep so we're fine in that respect. What I'm going to tell you is my method, a professional will say it's taking far to much time to do it like this and he's absolutely right from a business point of view. On the other hand, works for me.

I'm a lousy sander, I do have a problem to just flatten the filler instead of cutting in to it. I take another approach, I fill the dent with somewhat more filler it needs and I don't fill the whole dent at once, just the lowest part of it. The next picture shows what I mean.


Then I start to sand it back so it's level with the bonnet. Mind you, at this stage the edges are not feathered out into the bonnet yet. The small spot was no problem, it turned out perfect after the first pass of sanding.


I normally apply a 2nd layer of filler and when necessary even a 3th one around the edges. when everything is perfectly flat it looks like this. Yes, I discovered another very slight dent so I filled that as well.


That concludes this chapter, preparation will be taking up most of the time you need to do the repair. I think I mentioned it before, there are a few things you need to know about paint:

Paint won't fill dents or scratches, average car paint is thinner than water. Yes, there are high-build primers/fillers but I won't discuss that in this section. It'll be in the section "Primers".

Be sure paint doesn't cover up any imperfections, if you didn't prep it right the most professional paintjob will look awful. Good quality paint isn't cheep, it's just wasting money if you spray it over a panel or surface which is not prepared like it should.

Last but certainly not least, if you're doing it yourself take pride in what you do! That means you have to put in the time to prep it right, yes I know it's boring to sand time after time but a good result is what you can show your friends and most important, you will be proud showing it off!

Note: Don't be afraid to start a repair. If you need to get something right so your car looks good again it can't get any worse as long as you're willing to put time and effort in it. If you won't, let a professional solve your problem. If this is what you want to do yourself then go for it!

× Prep work Sanding Repairs

Last update : August 24th, 2018

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