Sanding/Finishing fins: with or without a sanding block?

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txlonewolf

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Hi folks. I'm working on improving my fin finishing techniques. I go for a 15ft finish (looks good/OK from 15 feet away), not perfection. My most recent endeavor was with Duplicolor gray primer/surfacer, brushed on, to be sanded smooth after drying. Rocket is an Estes Executioner, with stock 1/8" plywood fins. I chose to (try and) get a primed/sanded/smooth surface on each fin prior to mounting it on the body tube. After fin attachment, I'll use rattlecan primer and then finish paint on the assembled fin/airframe unit.

As a kid, I used sanding sealer and then sanded fins by hand, not using a sanding block. I usually went for three coats, sanded after each coat. On my Executioner, I tried sanding the flat fin sides with sanding blocks: an Apogee Sanding Tee and a 3M rubber sanding block I had on hand. My thought was that I'd be more likely to get a smooth, flat surface with the assistance of a flat tool than I would if I just held the sandpaper in my hand/fingers. I used 3M, 320 grit Pro Grade Precision sandpaper (comes in a puple-ish package). I tried using both linear and circular sanding motions as I worked. I sanded each bare fin side lightly, then brushed on the primer, let it dry, and then sanded again using the sandpaper and block(s).

While these came out OK, I found that I accidentally rounded the surface down at the leading and trailing edges of the fin. I attribute this to letting the sanding tool run too far over each edge of the fin, thereby levering the block down over the edge. I was applying pressure on the tool after it ran past the fin, though I did not do this on purpose. I was careful to not apply too much pressure, and let the sandpaper do the work. Pics of one representative fin are below. Root edge is taped off with masking tape to keep the primer off of surfaces that will be glued later. You can see where I unintentionally sanded all the primer off of the leading and trailing edges.

So my question to the experienced: When sanding the flat areas of fins, do you use a tool or just your bare hands? Any suggestions/advice will be appreciated.

Pics:
1_WholeFin.JPG
2_TrailingEdgeRounded.JPG
 

David Schwantz

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I employ all 3 types. Fingers, block and a mouse. I find each has it use. Large imperfections I use a wooden block 18" long and just wide enough to wrap a full sheet all the way around. That way I get two large sanding sides and one narrow one. For what you are showing, I would use a foam block, smaller and able to have finer control. Also use foam block for all of my wet sanding. Fingers are great for finishing the edges and concave surface of fillets. But after 60 years, I like fingers most. But for the mouse, I buy the smallest one I can find. Easier to control, and will get into tighter spaces.
 

Bruiser

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I just use my fingers for sanding primer and paint on rockets.

-Bob
 

Spitfire222

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In your example, the effect you're describing seems minimal, and you'll likely improve with practice. One thing I've learned over the years is to take your time and check your progress often. If you notice rounding or other unwanted results, adjust your position/technique, and try again. As the saying goes, it's easy to remove/sand it off, but more difficult to put it back on.

Personally, I use lots of different methods for sanding. I have lots of sanding blocks, sanding sponges, and do a lot of hand sanding. I find it very useful to make custom sanding blocks from scraps of wood using spray adhesive. Another technique I sometimes employ is to prime and sand the fins before removing them from the stock wood to eliminate the chances of rounding the edges. All that being said, your results look satisfactory, so I wouldn't worry too much about your technique; it will likely naturally improve with practice. Good luck!
 

Ihocky2

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Whether blocks, sponges, erasers, foam, you should use some sort of backer instead of your fingers. Your fingers are too soft and conform to any imperfections. Sand a piece of pine with just your fingers and it cuts through the soft areas, leaving the hard grain rings higher. If sanding a run in the paint you will sand down the paint around it before the run.

While there is a lot more information than really needed for rocketry in it, get Bob Flexner's book on wood finishing or finishing. He explains prep work and materials very well. He explains the different types of finish and methods of applying. The key to a great finish is all in the prep work.
 

txlonewolf

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I employ all 3 types. Fingers, block and a mouse. I find each has it use. Large imperfections I use a wooden block 18" long and just wide enough to wrap a full sheet all the way around. That way I get two large sanding sides and one narrow one. For what you are showing, I would use a foam block, smaller and able to have finer control. Also use foam block for all of my wet sanding. Fingers are great for finishing the edges and concave surface of fillets. But after 60 years, I like fingers most. But for the mouse, I buy the smallest one I can find. Easier to control, and will get into tighter spaces.
Thanks, David. Foam block: Is that the same as a sanding sponge? Or something different?
 

txlonewolf

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In your example, the effect you're describing seems minimal, and you'll likely improve with practice. One thing I've learned over the years is to take your time and check your progress often. If you notice rounding or other unwanted results, adjust your position/technique, and try again. As the saying goes, it's easy to remove/sand it off, but more difficult to put it back on.

Personally, I use lots of different methods for sanding. I have lots of sanding blocks, sanding sponges, and do a lot of hand sanding. I find it very useful to make custom sanding blocks from scraps of wood using spray adhesive. Another technique I sometimes employ is to prime and sand the fins before removing them from the stock wood to eliminate the chances of rounding the edges. All that being said, your results look satisfactory, so I wouldn't worry too much about your technique; it will likely naturally improve with practice. Good luck!
Making sanding blocks from scrap wood and spray adhesive: great idea! Priming and sanding before removal from stock wood is also something I'll try. Thanks also for your kind words.
 

David Schwantz

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Kind of. A sanding sponge has grit on it. A foam block just has sandpaper wrapped around it. I like the foam block better as the corners will not dig in when you are looking some place else. Although, I do use the sanding sponge at times. All just kind of depends on what and where I am sanding.
 

txlonewolf

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Whether blocks, sponges, erasers, foam, you should use some sort of backer instead of your fingers. Your fingers are too soft and conform to any imperfections. Sand a piece of pine with just your fingers and it cuts through the soft areas, leaving the hard grain rings higher. If sanding a run in the paint you will sand down the paint around it before the run.

While there is a lot more information than really needed for rocketry in it, get Bob Flexner's book on wood finishing or finishing. He explains prep work and materials very well. He explains the different types of finish and methods of applying. The key to a great finish is all in the prep work.
Thanks, Ihocky2. Your comment on fingers being too soft is exactly why I gravitated to a block. I'll look into Flexner's book. Too much info is way better than not enough. :)
 

txlonewolf

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Kind of. A sanding sponge has grit on it. A foam block just has sandpaper wrapped around it. I like the foam block better as the corners will not dig in when you are looking some place else. Although, I do use the sanding sponge at times. All just kind of depends on what and where I am sanding.
Got it. Thanks again.
 

dr wogz

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I prefer a sanding bar (a hard base) to ensure flatness & evenness over the area. Sanding sponges & foam / foam backed blocks will conform to dips & dimples & softer areas.

Sanding pressure and motion also play into a nice sanded surface. Long smooth strokes.. Be conscious of your arm movement. A lot don't realize, but as you extend your arm, we tend to add a twist in the motion / movement! (Ask any bowler or pool player!) Its just how are bodies are made. Knowing this, helps you keep a straight flow / motion.

I was lucky enough to grab the Great Planes sanding bar when I could! (They are rare now) but any 'nice & flat' hard sanding block works.

 

teepot

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I wouldn't worry about rounding the edges. It just makes the fin more aerodynamic. I use a belt sander and an orbital sander to make fins. I try to do as little hand sanding as possible. But when I do I use a block.
 

DES

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Hi folks. I'm working on improving my fin finishing techniques. I go for a 15ft finish (looks good/OK from 15 feet away), not perfection. My most recent endeavor was with Duplicolor gray primer/surfacer, brushed on, to be sanded smooth after drying. Rocket is an Estes Executioner, with stock 1/8" plywood fins. I chose to (try and) get a primed/sanded/smooth surface on each fin prior to mounting it on the body tube. After fin attachment, I'll use rattlecan primer and then finish paint on the assembled fin/airframe unit.

As a kid, I used sanding sealer and then sanded fins by hand, not using a sanding block. I usually went for three coats, sanded after each coat. On my Executioner, I tried sanding the flat fin sides with sanding blocks: an Apogee Sanding Tee and a 3M rubber sanding block I had on hand. My thought was that I'd be more likely to get a smooth, flat surface with the assistance of a flat tool than I would if I just held the sandpaper in my hand/fingers. I used 3M, 320 grit Pro Grade Precision sandpaper (comes in a puple-ish package). I tried using both linear and circular sanding motions as I worked. I sanded each bare fin side lightly, then brushed on the primer, let it dry, and then sanded again using the sandpaper and block(s).

While these came out OK, I found that I accidentally rounded the surface down at the leading and trailing edges of the fin. I attribute this to letting the sanding tool run too far over each edge of the fin, thereby levering the block down over the edge. I was applying pressure on the tool after it ran past the fin, though I did not do this on purpose. I was careful to not apply too much pressure, and let the sandpaper do the work. Pics of one representative fin are below. Root edge is taped off with masking tape to keep the primer off of surfaces that will be glued later. You can see where I unintentionally sanded all the primer off of the leading and trailing edges.

So my question to the experienced: When sanding the flat areas of fins, do you use a tool or just your bare hands? Any suggestions/advice will be appreciated.

Pics:
A shorter sanding block, approx. 6 inches long or so gives better control, and the ability to "stay off the edges" when sanding. You can more or less focus on the middle, then go around and focus on the outer rim and smooth out the edges. Whereas a long sanding block is going to be constantly cutting on the edges. Also, you have to take extra care to go light on the edges. The sanding block is always going to want to cut fast there, because as it runs off the edges, the block is unsupported and effective pressure goes up. Hold the block toward the back half, so your hand is always over the fin and never runs off. Don't apply force, let the weight of the block do the cutting.

A good technique for smaller fins is to prime and sand the flat sheet before you cut the fin out; that way the larger sheet supports the sanding block and prevents dig in on the edges. For bigger fins, you can back up the edges with scrap.

The other thing - grit selection. 320 works, obviously, but takes longer, so now you have more passes over the edges. For dressing off the first coat of primer, something like 180-220 is fine, cuts faster, so 3 or 4 light swipes of the block get you level, without overworking the edges. Then you can prime again, and very good chance that coat won't require any sanding.

But all that said most people will deliberately break the edges and round them over anyway to reduce drag.
 

txlonewolf

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Thanks, all. Really helpful tips. I've saved these and will incorporate them in future projects.

Happy flying (and sanding)!
 

icyclops

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In the past I just used my hands and a piece of sandpaper....Now days I laminate card stock paper over the fins using spray mount 77 and seal the edges with white glue, I don’t sand anything....ready to go to primer....why would you sand when lamination/paper cover is so much easier....and done properly will hold up better, stronger than plan balsa/sealer?
 

txlonewolf

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In the past I just used my hands and a piece of sandpaper....Now days I laminate card stock paper over the fins using spray mount 77 and seal the edges with white glue, I don’t sand anything....ready to go to primer....why would you sand when lamination/paper cover is so much easier....and done properly will hold up better, stronger than plan balsa/sealer?
I, too paper my balsa fins. I use the technique that Luke Strawwalker documented here. I'm looking to expand my skillset to include a nice finish on plywood fins using primer, sanding and paint. I could have papered these, but chose instead to work on a different technique.

Cheers!
 

cbwho

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In the past I just used my hands and a piece of sandpaper....Now days I laminate card stock paper over the fins using spray mount 77 and seal the edges with white glue, I don’t sand anything....ready to go to primer....why would you sand when lamination/paper cover is so much easier....and done properly will hold up better, stronger than plan balsa/sealer?
What weight card stock do you use? What weight can ink jets handle?
 

caveduck

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Various ideas, I've used these all with good effect. These days I always try to use a machine or tool if at all possible. My plywood fins usually either get glassed or just a resin coat (West 105/205) to seal them up and make a good base surface. Smaller balsa fins get sealed with super glue.
  • Tape sandpaper down to a cheap glossy ceramic tile. Very flat and indestructible in normal use. You almost cannot unintentionally round the corners this way.
  • Spray adhesive sandpaper onto an aluminum extrusion, bar, T-shape, whatever. Or wood blocks but they are not always dimensionally stable.
  • Use a Dremel Multi-Maxx oscillating sander. This is fantastic for a lot of MPR/low HPR sized flat things. You can bevel fins with this if you want.
  • Use a 5" orbital sander for even bigger flat stuff.
  • slightly OT: Glue sandpaper to brass tubes of various sizes to sand fillets
 

txlonewolf

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Various ideas, I've used these all with good effect. These days I always try to use a machine or tool if at all possible. My plywood fins usually either get glassed or just a resin coat (West 105/205) to seal them up and make a good base surface. Smaller balsa fins get sealed with super glue.
  • Tape sandpaper down to a cheap glossy ceramic tile. Very flat and indestructible in normal use. You almost cannot unintentionally round the corners this way.
  • Spray adhesive sandpaper onto an aluminum extrusion, bar, T-shape, whatever. Or wood blocks but they are not always dimensionally stable.
  • Use a Dremel Multi-Maxx oscillating sander. This is fantastic for a lot of MPR/low HPR sized flat things. You can bevel fins with this if you want.
  • Use a 5" orbital sander for even bigger flat stuff.
  • slightly OT: Glue sandpaper to brass tubes of various sizes to sand fillets
Thanks for the ideas. Glassing is on my to-learn list. :)
 

ep29030

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Making sanding blocks from scrap wood and spray adhesive: great idea! Priming and sanding before removal from stock wood is also something I'll try. Thanks also for your kind words.
I also make sanding blocks from pieces of scrap wood and 3M adhesive. I make them in various grits: 60, 100, 200, 320, 400. They work great for getting flat surfaces. But as the finish gets closer, I usually switch to hand sanding, so I can work areas that need a little more attention. 2-3 coats of sandable primer, sanding between coats. My philosophy is to sand/primer, then after two paint coats, I do wet sanding, 800-1000 grit, then apply decals (optional), then 2 coats of clear. This gives a nice finish, and protects the decals. Everyone as their own preferences, but that is my approach. Mostly use rattle cans and get good results, but I have no use for Rustoleum sprays. Too many bad experiences and clogged cans. Mostly use cans from auto parts stores.
 

jqavins

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Most of the fins I've sanded are balsa. A few plywood and one set was basswood.

I use my favorite lightweight filler on balsa while it's in the stock sheet, as others have said.

I have only two sanding tools for the vast majority of what I do. I'm sure I would benefit, now and then, from some others. But I do just fine for a 6 foot finish with just these two: 1) fingers for most rounded places; 2) a 5 inch long piece of oak S4S 1×4 for flat places. Wrap some paper around the block, tape it if necessary, and get working. A 2×4 might be more comfortable in my hand, but the store didn't have any oak S4S 2×4 on the day I bought what I've got. When I start doing much larger fins I may well switch to a mouse of palm sander, and I'd want to make sure I have one with a hard, flat place.

I always keep light to moderate pressure over the work piece. To get to the edges I let the block and paper extend over, but keep the pressure over the piece so the block remains level, never dips.

Often I just put my sandpaper face up on the desk and move the fin around on it. The main drawback to that method is sanding my finger tips. ;)
 

icyclops

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What weight card stock do you use? What weight can ink jets handle?
Very heavy card stock...the weight would be used for tabs...very thick. Not shure of the weight....possibly 120#...is that close.
 
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