Sanding ?

[Christian1971]

Does the plastic model car go through the sanding process before or after it gets the primer?
Why go through different sand paper when sanding rather than just fine sandpaper?
When sanding should the entire car get sanded thoroughly during each "sanding stage"?
Thank you!!!

[hirofkd]

You're talking about model car bodies, right? Or I assume that's your primary focus.
1) Preferably both, because body parts have imperfections, like sink marks and parting lines. You may also sand a body when it's primed. It helps the body paint lay smoother in the later stage of painting.

2) Fine sand papers take too long to smooth, while coarse sand papers leave nasty scratches you don't want to see when your model is finished. That's why you use various grades successively.

3) Preferably yes, but not extensively. The idea is to have uniformly smooth surface all around the body. After applying body color, some areas might turn out smooth, while the others might not. You can sand and polish only the rough areas. It's up to what level of perfection you want to achieve. Some modelers don't even sand or polish painted bodies.

It's just a hobby, and it's your model. Take it easy and enjoy, which I think is the most important.

[Christian1971]

For the plastic car model body (I am trying an f1 model) what numbers sandpapers would you recommend using one after the next? Go from coarse to fine?

[freakray]

Please don't start multiple threads for questions relating to the same topic - threads merged.

[cyberkid]

For modeling uses, the common grits are:
For shaping: 180, 240, 320, 400, 600, 800, 1000, 1200, 1500, 2000
For polishing: 1500, 2000, 2400, 3200, 3600, 4000, 6000, 8000, 12000

The higher the number, the finer the grit. I normally don't go under 800 when working on plastic models. 180, 240, 320, 400, 600 grit are normally used for shaping scratch built mods. When starting out, you only really need : 800, 1000, 1200, 1500, 2000 (In that order). The higher grits are optional, some people use them, some people don't.

Search a little on 'wet sanding' and 'compound'. And most of all try it! Most of what you learn is by doing it.

[Christian1971]

I apologize for all my questions. I have read the faq and 'how to' but still unclear. When sanding, can I apply one layer of primer, then sand with next higher grit, prime again, the use the next higher grit and so forth? Washing away dust after each sanding.

[freakray]

Merged threads again - please don't make me merge a third thread on the same continuing topic.

And to answer your question, if that is what you want to do, yes it is possible to do it. Why would you want to do that though?

[RallyRaider]

Just use the finest sandpaper that will do the job. For sanding after priming all you'll need is 1500 or higher to knock the edge off. Unless there is a big ugly blemish you need to correct.

I'm afraid that most basic modelling techniques are best learnt by jumping in and using your common sense. Simplicity is best.

[willimo]

I think most people sand their bodies following the same steps I do. This is what I do:

1. A courser sanding to get rid of any mold lines or artifacts of the molding process. This is usually limited to any mold lines around the body, most noticeably around the front and rear bumpers (but often continuing over the roof), ejection pin marks, or sinks.
------1b. Occasionally, if I'm really getting down to business or if the body is bad, I will use a misted layer of primer before sanding, to help locate big time sinks, and fill them with putty.

2. A once over with a finer grit, to help smooth all the scratches and whatever else left by my clean-up sanding. I'll sand the whole car with this, not just the problem areas, in order to give the car a smooth surface that's, paradoxically, rough enough to easily accept primer (not everyone agrees with this, and most people would just smooth the areas affected in step 1)

3. Primer. Just spray your favorite primer in your favorite way. Give the model a look over and if there are problems, repeat 1, 2 if necessary. Don't repeat if not necessary!

4. Paint. Too late! If there are any body mistakes here, too late! The only sanding you will do at this point will be wet sanding the paint (after it has cured!) to get rid of any blemishes in the paint. Usually I won't use anything greater than 2400 grit here, typically use 3600 and then a polishing compound to restore shine.

Plastic model kits are really not very sanding intensive at all. The only real aspects we have to get into are fixing molded in mistakes, or mistakes we make a long the way. It's a straightforward process, I applaud your effort to research it, but don't make it overcomplicated! Let it be fun, and remember, do only what you want to do!

[Didymus]

Here's my take on sanding after painting.

(Do I have this right? I'm pretty new at this, so I don't claim to have any expertise, but this is what I'm picking up from personal trial-and-error and from reading the forums. I'm a writer by trade; writing things down helps me clarify my thoughts.)

Sanding color and clear coats is risky business. It's very easy to accidentally sand right through to the plastic or resin. Then you have to go back and either re-paint or touch up with a brush. Arrrgh. Sanding with multiple grades is also boring, frustrating and time consuming, partly because of all the backtracking that's needed to eliminate scratches from coarser grits.

So it seems that the trick is to use a painting method that will MINIMIZE sanding after applying the last coat(s) of paint, whether or not clear coat is used. Apply the smoothest possible final coat of paint, then start with the smallest grit or the finest compound that will efficiently remove orange peel, unevenness and other defects. After that, use progressively finer grits/compounds to get the finish you want.

No matter how smooth the final paint coat, I don't like the look of uncut glossy paint. It looks too plasticky.

(I know a professional modeler who NEVER sands after the final color or clear coat - and gets great results. He uses only compound, then wax. Of course, he's a master painter.)

Depending on the smoothness of your final coat, you could begin finishing with grit as coarse as 1500, but finer is better if it can do the job efficiently. I prefer compound to paper because it gets into the nooks and crannies. Then you work your way (down in grit size, up numerically) to something like Meguiar's Show Car Glaze, then, after the model is complete, finish with wax.

If the surface is so rough that you need to start with a grit that's coarser than 1500, your paint hasn't been applied smoothly enough. It takes forever to remove the scratches made by 800 grit paper. Even 1500 is a long way from a true gloss. As I said, the multiple sandings/rubbings which would be necessary even after 1500 greatly increase the risk of sand-through. And of course there's the tedium problem. Ideally, your paint finish is smooth enough to enable starting with Dupont #7 compound.

So the key to a good paint job is not sanding the paint, but in learning to apply a smooth, wet-looking final paint surface that doesn't hide detail. Then finish with compound, or at worst, a series of sandings with the finest grits. For me, learning to apply paint smoothly has been hard. It takes a lot of practice and experimentation with viscosity, air pressure and brush handling. But I've also learned that it can be done.

Comments?

Didy

[stevenoble]

Hi Didy,

If you concentrate your efforts on getting the best possible finish on your paint jobs there should be no reason to sand at all.Just using polishing compounds and a final wax will be all it needs to bring out the best shine.The only time I would sand is between coats of colour and only then if I had a reason to do so,say for instance if you got some dirt nibs or a small hair etc in the finish.My general rule of thumb is to use the finest grit you can get away with using to get the job done.Use of coarser grades will put tiny scratches in the surface that will be hard to remove properly.The most common grades I use are 800,1200,1500,2000 and Micromesh 6000,8000,12000.I wouldn't recommend taking 800,1500 or even 2000 to your final top coat of paint,they are too rough in my opinion.If you get the painting right in the first place you shouldn't need to sand it.The key to applying 'wet' coats of paint is the magic moment just before it runs.It's wet enough for a great finish but not too wet (it runs) If you put the paint on too dry you'll also get a very poor finish.There are a lot of variables.Thinning,air pressure,distance between the airbrush and the surface you are spraying all play a part.It's a case of practice makes perfect.Get yourself an old model body and practice,practice,practice.Good luck with it anyway.

Best regards,

Steve.

[Didymus]

Steve, what do you do if you get a sag in the color coat? Is it into the CSC vat, or would you use the 1500 just for that area?

Didy

[MPWR]

It really just goes to show there are many ways of going about it, and no one way is right. Each builder will develop and use his/her own technique.

I sand bare bodies with sanding sticks, so I don't really know what grit I use. I like to sand the whole body with a medium grit. Nearly every square cm will be abraided. Part of this is for tooth for the primer, but even more it's to ensure I have an even level surface. I probably use 600 grit last before priming.

I prime with a 'high build' acrylic laquer primer. It fills in minor scratches, so I don't worry too much about getting the plastic smooth first. After the first coat of primer, the real work begins. I use whatever grit stick or paper I need to address any issues with the primed body- often I will apply primer five or more times to a body (in different areas) depending on what needs fixed. When I'm really happy that there are no flaws, the primer gets 'polished' with fine paper- 1500 to 2000 grit. The primer gets extremely smooth and almost shines after this step- this is the ideal surface to apply paint to.

I apply color coats to solid opacity. More than that is not necessary. Any debris or flaws are addresed between layers, with very fine paper or sanding sticks. I control surface texture with my spraying technique, adjusting thinner and pressure as necessary. It's always best not to have to deal with texture afterwards- and once it starts, subsequent layers will build the texture. If it does appear, it gets addressed with fine paper/sticks in between coats.

Clearcoat is much the same as color coats- it's important to control texture as it happens. I'll do 10 or so light layers, again addressing any debris or flaws as they appear.

Done. The paint job is either terrific at this point or it's not- but all the real work has happened. Sandpaper of any type doesn't touch the body after the final layer of clearcoat.

I polish the clearcoat (after a week) with Tamiya compound and McGuire's ScratchX, and then wax it.

It took me alot of time and practice to develop this technique, and it works great for me. Doing it this way gives me results far better then I've ever gotten with micromesh. Personally, I hate micromesh for sanding paint (too easy to burn through and too hard to get those damn scratches out). But everyone goes about it their own way.

[stevenoble]

Originally Posted by Didymus

Steve, what do you do if you get a sag in the color coat? Is it into the CSC vat, or would you use the 1500 just for that area?

Didy

I wouldn't strip it just for a run in the colour coat.Only strip things when there's no other way around it as a last resort.Just address the problem area with a light sanding 1500-2000 used wet would be ideal for this.