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Old 11-07-2011, 07:42 AM
bobdros bobdros is offline
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Default Need advice for wet sanding

I've read about finishing techniques to learn how make a finish nice and glossy. One thread was about wet sanding the primer to a glossy finish but I didn't understand how that would affect the finishing coats. Why would you wet sand the primer coat?

I just prime to prepare for the final coats. Why would you sand it at all?

Does wet sanding actually make a finish glossy? (My Der Grosser Vati turned out pretty nice but it's not as glossy as the one on Excelsior's site.) Can you tell me more about how wet sanding makes the coat of paint glossy?
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Old 11-07-2011, 09:35 AM
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Simply put, the better your prep work the better your final paint will appear. That's the long and short of it.

I wouldn't particularly call what I do "wet sanding"... more like "damp sanding". The only purpose of the water is to prevent the paper from gumming up. I usually put about 2-3 coats of primer on the rocket, then sand it with 220 grit, sanding the primer down as smooth as I can get it. If I can see ANY imperfections that need more primer, it gets another couple coats of primer then and there, focusing especially on the "bad spots". Then it gets sanded down with 220 again. When it's as smooth as I can get it with 220 grit, I switch to 440 wet/dry paper.

Now, the finer the grit, the less material the paper takes off, and the easier it is to clog. 440 grit clogs pretty darn easily, but if you use a bit of moisture to keep the paper damp, it won't hardly clog at all. I take an old towel and a bowl of water and a few paper towels and usually sit and sand while I watch TV. Dip the paper in the bowl, shake the excess back into the bowl, and gently start sanding, in small overlapping circular motions, gently turning the rocket in your other hand as you sand. This will virtually eliminate sanding scratches. As you sand, the moisture will combine with the sanded-off primer to form "sanding mud" (literally just like mud) which you'll periodically (usually say every 3-4 minutes) wipe off with a damp paper towel. The main thing is, don't press too hard-- let the paper do the work. As the sanding mud takes up the moisture and the paper dries out, you'll soon see the sanding getting less and less effective... time to dunk the paper again. I rub my thumb over the paper underwater to release the stuck paint particles from the grit. Dunk it a time or two to 'wash off' the paint particles, shake the excess water off, maybe daub it lightly on the towel to remove excess water (the paper should be thoroughly damp but NOT dripping water everywhere! This is how it's different than "wet sanding" like you would with a car or other metallic or waterproof object where oftentimes you sand with a water hose dribbling to flush the paint particles away and keep the paper open-- rockets don't need or like that much water! ) I usually wipe the mud off at about the same time. The points to remember for excellent sanding results are 1) ALWAYS sand in small circular motions, even up to and around protrusions like launch lugs and fins, NEVER sand in a straight line if you can possibly help it, 2) don't press too hard or sand too fast-- let the paper grit do the work and remove material at its own speed, and 3) NEVER sand in only one spot-- keep the work piece slowly moving by turning it in your hand, or constantly moving to a different area up or down the tube or fin.

Once you've damp sanded the entire rocket, wipe it down thoroughly with a damp paper towel again and dry it with another paper towel or a rag to get rid of any remaining sanding mud/dust. Then hold the rocket up to the light, and sight along the tube. Rotate the rocket slowly with the light "glint" reflecting off the tube-- you should see a smooth, steady, unbroken reflection of light-- if the reflection is jagged or broken, or if there are any pits or imperfections, they'll be glaringly (pun intended) obvious in the glint of reflected light off the tube, nosecone, transition, or fin surfaces of the rocket. Then you can do a bit of remedial sanding in those areas with the damp 440 grit again, if necessary (hopefully you don't have to add more primer-- usually by this phase, if you've done a careful priming and 220 grit dry sanding job, you won't... any minor imperfections at this stage will be small enough that damp sanding can eliminate them.) If you get a perfect reflection, you'll have a perfect surface to which to apply paint, and if you apply the paint correctly, you'll have a perfect paint job!

You cannot make up for poor surface prep with the paint job, plain and simple. Any imperfections in the surface are MAGNIFIED by the paint, not covered by it! That's why if I can see ANY imperfection in the reflection I go after it again with damp 440 grit until the reflection is 'perfect'. It sounds involved and like it takes a long time, but it really doesn't... when you get down to imperfections tiny enough that you can't even feel them, that you have to SEE them in the reflected light off the tube/surfaces, usually you can remove them with just about 5-10 seconds of further sanding, a wipe with the paper towel, and return to the "glint inspection". You shouldn't see ANY sanding scratches or other things-- a little bit of work should get rid of them all, but you shouldn't hardly have any as long as you're sanding in small circles and not sanding in one spot for more than a second or two AT MOST...

Let the primer dry thoroughly (usually overnight) before painting. You can actually make primer shine almost like a finish coat by this process... A primer coat, straight out of the can, is INTENDED to be sanded. VERY FEW primers are actually meant to lay down smooth and glossy without a sanding. I've had people say "well if the primer's glossy, it won't have enough "tooth" to hold the final coat of paint-- it'll peel off" I've never had that happen, and I've been doing this for 4 years or so. On a microscopic scale, primer is still composed of minute particles and has tiny holes and voids in it-- it doesn't "flow out" into a single unified unbroken "skin" like paint does. That's why cars that are primered and driven for months usually end up rusting badly under the paint when it's applied-- rain soaks into the primer. I've also had people say "see, you should NEVER wet sand primer-- the water will "soak into" it and be trapped under the paint, causing problems. Cars are wet sanded all the time-- the key is, let the moisture dry out thoroughly (overnight for our rockets in a fairly low-humidity environment (say inside an air conditioned house) is usually enough in my experience). Paper tubes and balsa don't rust (though they can swell or warp, hence why you only use a VERY SMALL amount of water to "dampen" the sandpaper and form sanding mud as you work). So long as the primer has had time to dry and 'air out' the moisture isn't a problem.

When you paint, the paint will "flow out" as the individual droplets of paint break their surface tension and flow together with the other droplets of paint out onto the surface. They will also flow into the microscopic pores and particles on the surface of the primer and then "lock" into place when the paint dries. If you've done a good surface prep job, the paint will flow out into a uniform skin of color, making the rocket VERY glossy and shiny (if you're using gloss paint, obviously-- matte paint actually stays somewhat particle-ized to break up the reflective surface and prevent the glossy, shiny reflective skin properties of gloss paint, much like the surface of primer right after priming.
I've NEVER had to color-sand a rocket to get a terrific finish using these methods... if the primer coat is silky smooth, the finish coat will be silky smooth, glossy, and beautiful (barring the paint acting up and "spitting" or something (bad can or nozzle) or application errors or problems during application).

One other tip-- NEVER paint with a can of cool/cold paint. ALWAYS make sure the can is AT LEAST 75-90 degrees before starting to paint-- don't take cans from air conditioned indoors straight outside and start painting-- leave them outside in the sun for a few hours to warm up thoroughly, or set them in a pan of hot tap water and let them soak up the heat before painting-- warm spray paint cans spray at higher pressure (more latent heat in the paint/can to boil off the aerosol propellant, therefore higher vapor pressure of the propellant in the can) giving better atomization and smaller droplets. Also, the paint is thinner and less viscous when it's warm, meaning it flows easier through the nozzle, atomizes better, and flows out better when it hits the surface of the rocket. Warm cans usually solve about 99% of the spitting or other problems rattle cans are prone to.

Later and good luck! OL JR

PS. If you want to see examples, just search for any of my build threads...
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Old 11-07-2011, 09:45 AM
bobdros bobdros is offline
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Now that's an explanation! Thanks! I've been using Rustoleum and Krylon enamels with so-so results. I've read that air brushing works better. Years ago I bought a Testors Aztek air brush but never really used it. I don't remember if it's acrylic or enamel but I seem to remember reading that acrylics are good to work with. Would they flow better than enamels? Any opinions?
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Old 11-07-2011, 11:41 PM
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luke strawwalker luke strawwalker is offline
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I've got a cheapy external mix TSC airbrush that I got last year but I haven't tried it yet.

I've heard that basically you have to "heat set" the acrylics to get them to work or they'll peel right off. I don't like that part of it very much.

You can spray anything through an airbrush if you thin it right... even automotive paint. Just a matter of using the right thinner in the right amount (probably a bit of experimentation required).

So far I've had good luck with the rattlecans so I'm slow to change...

Later! OL JR
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Old 11-07-2011, 11:44 PM
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luke strawwalker luke strawwalker is offline
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Quote:
Originally Posted by bobdros
Now that's an explanation! Thanks! I've been using Rustoleum and Krylon enamels with so-so results. I've read that air brushing works better. Years ago I bought a Testors Aztek air brush but never really used it. I don't remember if it's acrylic or enamel but I seem to remember reading that acrylics are good to work with. Would they flow better than enamels? Any opinions?


Rustoleum is pretty good paint, but the new Krylon leaves a lot to be desired IMHO. Valspar is a good brand, and I've had EXCELLENT results with the 99 cent a can Walmart Colorplace stuff, believe it or not.

New Krylon seems just a lot more finicky to get it put on right and get it to turn out right. It'll work, no doubt, but it's sorta like "if everything's PERFECT then MAYBE it'll turn out ok..." I don't like that. I like simple, dependable paint that does a good job 98% of the time. The Walmart Colorplace is actually that paint... it's everything new Krylon isn't... Rusto's sorta "in between"...

All in my experience...

Later! OL JR
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Old 11-08-2011, 09:18 AM
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NOTHING and I mean NOTHING beats the finish of good old fashioned colored Aircraft Dope shot out of an Airbrush short of Automotive activator-required-URETHANE out of a paint gun.
Rattle cans cannot come even close without a bunch of color sanding.
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Old 11-08-2011, 11:12 AM
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Quote:
Originally Posted by ghrocketman
NOTHING and I mean NOTHING beats the finish of good old fashioned colored Aircraft Dope shot out of an Airbrush short of Automotive activator-required-URETHANE out of a paint gun.
Rattle cans cannot come even close without a bunch of color sanding.


I dunno, GH - I've seen some really excellent results with high quality flat acrylics applied with an airbrush. I was won over to acrylics from enamels eons ago (back in my IPMS days) - of course you can't use dope on plastic models so acrylics, in my view at least, are always the best choice. My biggest problem with dope is its decreased availability. Back when I started building rockets and balsa/tissue airplanes, dope was easy to get and came in. Lot of different colors. Now, it seems like all I ever see are clear and sanding sealer. Yes, I could mail order it, but I like buying my finishing supplies locally. And I'm happy enough using acrylics and can get them in my LHS, so it's all good.

Greg
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Old 11-08-2011, 11:53 AM
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ghrocketman ghrocketman is offline
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For dope finishing, one must be a little more resourceful, but in my book that is the price to pay for a superior finish.
For butyrate dope, your one-stop-shop is Brodak.com
They have at LEAST twice the colors ever offered by Aero Gloss.
GOOD independent hobby shops carry it.
I have two within 7 miles of where I work.
I HATE with a passion all non-solvent based finishing systems; the ONLY good acrylic paint is Acrylic LACQUER !
I would rather spray with rattle-can enamel than ANY water-based junk.
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Old 11-08-2011, 11:57 AM
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Quote:
Originally Posted by ghrocketman
NOTHING and I mean NOTHING beats the finish of good old fashioned colored Aircraft Dope shot out of an Airbrush short of Automotive activator-required-URETHANE out of a paint gun.
Rattle cans cannot come even close without a bunch of color sanding.


Can't say I can fully agree; I think rattlecans and airbrushes can each deliver essentially the same result with proper surface prep. I think the real differentiator is in other areas. Rattlecans offer convenience, speed, and minimal clean up. Airbrushes offer a far broader color palette, and a degree of control with accent and shading that's tough to replicate technique-wise with rattlecans. And the clean up always seems to take longer than the time spent actually spraying.

On the left, all colors from rattlecans (with some judicious masking, of course). On the right, an airbrushed nose cone.
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Old 11-08-2011, 12:13 PM
raohara raohara is offline
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GH, thanks for the tip about Brodak. I didn't realize anyone sold butyrate dope anymore.

- Rich
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