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Sanding after applying paint

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Murrill

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I am planning on painting a checkerboard pattern on a rocket I am presently working on. This will require quite a bit of masking after I have primed the rocket. My concern is that whenever I mask, there is inevitably a little "ridge" where the paint surface is uneven between the two painted areas. With a checkerboard pattern, the "ridges" are going to be everywhere. My question is, can I sand down these ridges after the paint has been applied, without scratching the painted areas? My typical painting technique is to apply several coats of primer, sanding between each coat with progressively finer sandpapers. But I have never sanded the paint job itself, I typically only sand the primer prior to painting. What "grit" sandpaper should I use to sand down the checkerboard ridges but not scratch the paint? Any other tips or techniques that I can use?
Thanks in advance!!!
John
 

vjp

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Murril,

When you peel the masking tape, after spraying the second coat, how soon are you doing this?

If you wait too long, you'll develop a ridge. If, on the other hand, you peel the mask while the second coat is only semi-dry, the edge will "flow" and flatten out a little. This will provide a clean, ridge-free edge.

Also, when you peel the masking, peel it back over the freshly painted area, so any "pulls" will lay over the newly painted area and not the adjacent undercoat.

The amount of time to wait is dictated by the exact paint formulation and drying speed, and should be determined empirically by practicing first on scraps, to get the technique right. But once mastered, you can get an edge that's extremely clean and ridge-free. I've found that 10 minutes is a good starting point to test with, adjust this more or less as drying conditions and drying speed vary.

Also, some paints are better than others, so try a couple of different brands to see what works. I am partial to the Rustoleum enamels, myself.
 

lalligood

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Good question. And the answer isn't as difficult as you might think.

First you want to "wet sand" using superfine grit 3M ScotchBrite pads (or steel wool), which can be had in the paint section of your local home improvement store. "Wet sanding" is getting the pad wet (they hold water almost like a sponge) & lightly rubbing in a circular motion. [NOTE: Be careful though--assuming that your tubing is kraft paper, you don't really want to get the unpainted part(s) of the tubing wet because it'll ruin the tube!] When you have a nice smooth surface, pat dry the tube with a lint-free cloth. Don't be surprised if you can see light scratches in the surface of the paint--you should see that!

Now to restore the polish... At your local Wal-Mart, visit the automotive section. What you're looking for is 3M Rubbing Compound. It's a greyish-black 8oz bottle for about $6. If you don't have any disposable cotton cloths, pick up some cotton shop rags in that same section of the store (~$2 for 10). Put a small dab of compund on one cloth & rub with medium pressure in a circular motion. Wipe away any excess with a second, clean rag. Repeat as necessary until you achieve the desired finish & shine.

All of this isn't hard, but it does require some work & patience. I think you'll see that it was worth the effort!

Enjoy :D
 

limd21

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First, I am not an expert here, so this is only based on what I've seen others do - some more input or research is probably worth your while. Also this is based on things like pinstripes and flame jobs for cars and motorcycles.

On a good custom flame or pinstripe job, there is no detectable ridge. If there is, it's a sign of a job that's not done to the "peak" of the artform. It may look good to 99% of the public, but hot rodders and custom bike fanatics are a pretty exacting bunch.

First, you need to get as clean a line as possible. Ordinary masking tape has too many irregularities to make this possible. The usual "blue" tape is better, and may be good enough for model rockets, but the bike and car guys use something called fine line striping tape. It's more flexible (good for making curves for those flowing flames!) and has a nice sharp edge and is completely smooth, so paint is less likely to seep underneath like it does with ordinary masking tape. My guess is that drafting tape might make a good substitute for straight line work (e.g. your checkerboard). Whatever tape you use, you'll need to be sure it's not so aggressive as to pull off your base coat.

Use your fingernail, a popsicle stick or some other tool to make sure the edge of the tape defining the border is consistently stuck down. Again, this is to prevent paint bleed underneath the tape.

Shoot the color in very light mist coats. The first should be just a light "fog". Build up only *just* enough to get complete color coverage so no base shows through. If you build any more than this, the paint ridge will be higher and harder to deal with. As soon as the paint is dry to the touch, remove the tape (an xacto is a good tool to use to "pick" up the edge of the tape.)

What you've done to this point is to put down the pattern (stripe, flame, checker) as neatly as possible, doing what you can to minimize bleed and excess build up. A lot of folks may wish to stop here and say it's "good enough".

If you want to go further and get a smooth surface, then more work is required. Shoot a couple of coats of clear finish on top of everything. This will "smooth" the paint ridges to make them less noticeable. Again, many guys will say this is fine and stop here.

Even at this point, however, the paint ridges of the pattern will "telegraph" through the clear - so it's not smooth yet. To got further, you need to wet sand the whole thing using wetted sandpaper. Be careful not to soak the exposed cardboard interior surfaces! Using a flat sanding block, start wet sanding with 400 grit, work up to 600 and then to 1200. Some go to 2000 but that's pretty extreme. Be very, Very, VERY careful not to sand through the pattern layer. It's ok to go trough the clear, but if you burn through the color pattern layer, you'll need to start the over again. What you've now done is sanded the telegraphed "humps" of the clear down to the level the valleys where there was no color pattern. I.e. the paint should be smooth and "flat" and "straight" - as the body guys call it. Also, you'll note that the gloss is completely gone. The sanding has produced a flat surface sheen. Don't panic - what you've done is only supposed to get the surface smooth. If it's not completely smooth, you need to choose between sanding some more - at the risk of burning through the color - or shoot another layer of clear to give you some more "build" to work with. It's more time, but less risky and if you've gotten this far, you've already got some significant time invested so to blow it here would be a shame.

After you've smoothed and wet sanded to your satisfaction (body guys take days, even weeks on this step). You can shoot on a final clear coat to give it a good shine and restore the "depth" lost during the wet sanding process (BTW, body guys call this process "color sanding").

99% of car body guys would stop here. If done right, it looks pretty darn good at this point and the paint finish from all volume car mfrs consider it done once the final clear coat dries. Even your basic Chevy looks pretty darn good at this point.

The other 1% who are going for that perfect concours finish would sand once again to remove the slight orange peel finish that is inevitable with even the best spray equipment, and then use power buffing equipment to polish the paint to its final gloss. That gives you an idea of how fanatical the show car crowd is.

Now, who's crazy enough to do the "whole" works for a cardboard model rocket? Though it's not me, I bet someone out there is!
 

KermieD

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Another bit: Make sure you're using acrylic paints. My own personal experience is that enamels don't deal well at all with sanding.
 

jetra2

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Originally posted by limd21
Now, who's crazy enough to do the "whole" works for a cardboard model rocket? Though it's not me, I bet someone out there is!
OOOHHH CARL!!!

:D :D :D

Jason
 
A

Austin

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Jetra...you suck!!! :)

but you are also probably right...

Carl
 
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