How to sand down a complex rocket

[neil_w]

My IRIS-T need a new paint job. My first attempt at painting did not come out well, but even more importantly the color is too dark. And so, I need to sand it down (not to bare wood, but enough to make a good base for the new topcoat). I'm nervous to start this: once I start, of course, there's no going back.

The problem is the rocket is fairly complex and has a lot of nooks and crannies. There are areas that are recessed (or, more accurately, surrounded by raised features). I don't know how to sand in those areas, since there's very little room to move the sandpaper. Also, the inside edges in many cases seem somewhere between difficult and impossible to get to.

Here's an example:

This is the best picture I have handy to illustrate what I'm talking about. The area in the red oval is about 2" long and maybe .75" wide, surrounded by raised features. There is almost no room to maneuver sandpaper in there. Also, I find that I can cause significant damage with the edges of the sandpaper if I'm not careful. Sanding the edges of the raised bands seems virtually impossible.

This rocket has many(!) more areas like this one. What's a good approach? Should I worry about getting the entire surface equally sanded, or just do what I can and move on? I don't want to spend a billion hours sanding, but I am willing to put in some effort to get this right.

 

[Charles_McG]

Portable sandblaster?

 

[cwbullet]

Tight spots suck to sand.

 

[neil_w]

Tight spots suck to sand.

And to paint. The biggest downside of LPR that I've encountered.

 

[cwbullet]

He is right, sand blasting might be an option if the rocket is fiberglass.

 

[neil_w]

It’s paper and balsa, and I’m not set up for sandblasting anyway.

 

[Gary Byrum]

This should prove to be a learning lesson for you. I'd be surprised if it already isn't. "Nook & cranny" areas need special attention. Like, pre-painting before installing? That too, can be a bugger, but the end results outweigh the difficulty you are facing right now.

 

[Jozef]

What type and brand of paint did you use? If enamel based, how long has it cured? If its enamel and fresh, you had better wait until it is totally cured. Make small sanding pads using the flat, chunky style pencil erasers. The flex. experiment with sandpaper types and grits. try 320 or even 220 on a flat area to see how it cuts. If it is enamel, the paper will plug. Wet sanding might be risky on paper and wood if you cut brought. In the future, on a rocket with complex features, do your filling at the part level. you can even do a light coat of primer and assemble after sanding off most of the primer...leaving only low spots filled. Do a search of the QCC Explorer. At least one builder shows some advance part preparation before assembly of the pods and fin sets, include light primer.

 

[neil_w]

This should prove to be a learning lesson for you. I'd be surprised if it already isn't. "Nook & cranny" areas need special attention. Like, pre-painting before installing? That too, can be a bugger, but the end results outweigh the difficulty you are facing right now.

Prepainting (or even pre-priming) was not possible for most parts on this build because they were actually constructed in place on the rocket. The only exceptions were the rear control fins, although they are also simple and wouldn't have justified pre-painting.

Nope, this build was done the way it needed to be done. My incompetence as a painter is an entirely separate issue. (BTW: am I the only one who thinks it's Homer Simpson's face behind that hand?)

What *could* have saved me here was testing out the color before painting the whole rocket. That didn't even occur to me; it looked fine on the cap, although now I look back at it and can see that it's obviously too dark.

 

[neil_w]

What type and brand of paint did you use? If enamel based, how long has it cured? If its enamel and fresh, you had better wait until it is totally cured.

It's Rusto 2x enamel. By the time I start sanding it will have been at least two weeks since application. In my experience with this stuff, that's plenty of time.

Make small sanding pads using the flat, chunky style pencil erasers.

Hmm, pencil erasers. That's an interesting thought. I'll see what I can round up.

Wet sanding might be risky on paper and wood if you cut brought.

I've had no problems wet sanding in the past. Obviously I will have to be careful near edges that will have a tendency to sand through. My goal here, however, is not to completely remove the paint, just to smooth it down in preparation for a new topcoat.

In the future, on a rocket with complex features, do your filling at the part level. you can even do a light coat of primer and assemble after sanding off most of the primer...leaving only low spots filled. Do a search of the QCC Explorer. At least one builder shows some advance part preparation before assembly of the pods and fin sets, include light primer.

As mentioned, this was not generally possible on this rocket. If you look at the build thread you'll see why.

In hindsight, there could have been one or two places where pre-priming *might* have been possible, but it would have been risky and brought other problems with it. If I had to do it again, I'd do it the same way.

But with the correct paint color.

 

[Jozef]

Sorry I could not be of more help. Read you build thread. That is a complex design and challenge to build. Though.... If you did it again with the same products and process would you not expect the same results, but in the color you wanted? If you are happy with that finish, just take a fine scotchbrite pad, knock off the gloss then respray.

 

[neil_w]

If you did it again with the same products and process would you not expect the same results, but in the color you wanted?

There were 3 correctable mistakes (other than wrong color):
1) The wings needed a coat of filler/primer, at least on the outer parts above the skirts. It would have been a pain but I could have done it if I realized how important it would be.
2) There is way too much excess glue on various parts of the body tube. In many areas I tried to sand it off but had no luck; often those were the same hard-to-reach areas as I'm worried about sanding now. Glue boogers are a common problem in my builds, I just need to try harder to keep things clean as I go, particular when filleting.
3) My painting technique sucks. I guess it is debatable whether this one should be called "correctable"; you'd think I would have figured it out by now. But I think I can do at least a little better on the next go-round.

 

[Amsterdam]

*This is a ridiculous an overkill method*

make a sander using a cheap electric toothbrush, the small round head variety.
Yank out the bristles, use double sided tape to stick sandpaper to the freshly bald brush, and go to town.
Rusto2x will probably fill the paper really fast so cut plenty of replacements.

 

[Gary Byrum]

*This is a ridiculous an overkill method*

make a sander using a cheap electric toothbrush, /snip/.

And a waste of money. I have one of those and I regret ever having gone the distance making it. Takes FOREEEEEEEVER to sand something with it. Couple of years ago I ran across a company at a woodworkers trade show that offered a kicka$$ little pensander that I bought for LW for C'mas. I wish I'd remember to get one for myself but you can't beat this. I tried one and was sold instantly. This is one of those times when you would have said, "Now I'm damn glad I got one of these". LOOK HERE

 

[Amsterdam]

That is one vigorous pen sander, very cool

 

[Nathan]

I would just lightly rough up the whole surface of the rocket with 220 grit sandpaper, and then if you have problem areas (like excess glue or paint runs) sand those areas more thoroughly. Then spray a coat or two of filler primer and sand that with 400 grit. Then repaint with enamel.

 

[neil_w]

This is one of those times when you would have said, "Now I'm damn glad I got one of these". LOOK HERE

OK now that is cool. That would probably be an incredibly useful tool for me.

I would just lightly rough up the whole surface of the rocket with 220 grit sandpaper, and then if you have problem areas (like excess glue or paint runs) sand those areas more thoroughly. Then spray a coat or two of filler primer and sand that with 400 grit. Then repaint with enamel.

Well, then I would have the problem of sanding the filler/primer. That's why I didn't apply a coat in the first place. Maybe a mistake.

Is it safe to apply filler/primer over enamel topcoat? I thought primer over topcoat was risky.

 

[Brainlord Mesomorph]

Is it safe to apply filler/primer over enamel topcoat? I thought primer over topcoat was risky.

I recently had a problem with paint seeping under tape, so I gave a light sanding w/ 200 then put fresh primer on top of 2 colors of Rusto enamel.

repainted fine. no problem.

 

[Nathan]

Is it safe to apply filler/primer over enamel topcoat? I thought primer over topcoat was risky.

That is how I did it when I repaired my Super DX3. It was originally painted with Duplicolor enamel. After the repair a sprayed Duplicolor filler primer (which is lacquer based) over the old enamel and then a new coat of Duplicolor enamel. It may make a difference that the original enamel was a couple of years old when I did the repair.

https://www.rocketryforum.com/showthread.php?62382-Repairing-cardboard-tube-damage

I also sprayed primer over topcoat when I repainted by Frenzy XL, although that was lacquer primer over lacquer topcoat.

 

[Steve Shannon]

I have one of those Fein tools. They’re pricey, but a terrific tool for rocketry. They come with a detail pad sander that would allow you to sand up to the edges of some of your details without rounding the square edges.
They’re excellent also for cutting out broken fins to replace them.
For the first time ever TRF isn’t allowing a screenshot from my iPad, saying it doesn’t support the jpeg file extension.

 

[neil_w]

I have one of those Fein tools. They’re pricey, but a terrific tool for rocketry.

I just looked at the Fein website and didn't see anything small enough to use in this sort of situation. Can you provide a link?

I'm thinking of asking for one of those pen sanders for Father's Day...

 

[Gary Byrum]

I would imagine the pensander would be excellent for getting to those little glue boogers I sometimes get at the base of a fillet or elsewhere, and would have come in real handy for jobs like in the pic. The leading edges of these fins are laminated to broaden the look (muscle it up) for a aesthetics. It was a bear to sand in the "V" corners. That triangle pad would have made it a breeze.

 

[Steve Shannon]

I just looked at the Fein website and didn't see anything small enough to use in this sort of situation. Can you provide a link?

I'm thinking of asking for one of those pen sanders for Father's Day...

Mostly I was thinking of the pads to use on flat areas, but for tight spaces would these profile sanders help?
https://www.tylertool.com/fein-6380...MI4eO85_LB2wIVT57ACh0Q2g9hEAQYGSABEgLZ1vD_BwE

 

[jdbectec]

Try one of those automotive sanding pens like used for touching up nicks in paint. Should be able to find one in any auto parts store and I believe even Wal-Mart carries them. About $5-6.

 

[vcp]

And a waste of money. I have one of those and I regret ever having gone the distance making it. Takes FOREEEEEEEVER to sand something with it. Couple of years ago I ran across a company at a woodworkers trade show that offered a kicka$$ little pensander that I bought for LW for C'mas. I wish I'd remember to get one for myself but you can't beat this. I tried one and was sold instantly. This is one of those times when you would have said, "Now I'm damn glad I got one of these". LOOK HERE

Cool tool. I remember seeing these years ago. Rotten that you have to buy the power supply extra - non standard plug. The variable speed would be nice but that power supply is $$. You can order this at Lowes but you still need to order the separate power supply. (Should be able to chop it up to use a common wall wart - but need to check.) Looks like this would be really useful for small printed plastic parts.

Micromark has the same thing for $70, that includes the power supply, but it doesn't appear to have a pluggable cord so there is no option for variable speed.

Several vids on utube about converting toothbrushes, but they all apprear to be short on showing if they're actually useful.

Ok, yeah, I've been looking around and you should be able to use a $5 wall adapter and a $6 speed control with the Proxxon.

 

[Brainlord Mesomorph]

I hesitate to say something so simple and cheap, but I keep a package of emery boards w/ my sandpaper and blocks

 

[Gary Byrum]

I hesitate to say something so simple and cheap, but I keep a package of emery boards w/ my sandpaper and blocks

Chances are, we all do.

 

[Brainlord Mesomorph]

Chances are, we all do.

that's why i hesitated

 

[vcp]

While looking at the Proxxon on Amazon, I found these sanding sticks. Expensive, but looks pretty slick.

 

[neil_w]

While looking at the Proxxon on Amazon, I found these sanding sticks. Expensive, but looks pretty slick.

Those look cool. I feel like I'd need to see a 3d view around them (or a video) to fulliy understand their geometry. Haven't tried searching Youtube yet.

In any case, I decided to dive in and see how much I could do with just plain old sandpaper. I will work as slowly and carefully as my patience will allow (I'm allocating weeks for this job, doing just a little bit each session). My first run has been pretty good; there are a few crevices I didn't cover well but they are probably OK. Also, despite my attempted gentleness, I did sand through to the wood in one or two spot on fin edges. I think when I'm done I'm going to do a targeted coat (just the areas I want) of primer, just to cover up the few small spots of bare wood. Long way to go before I get there though.