Fins and filler

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Bone Daddy

Well-Known Member
Oct 3, 2009
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I've noticed that some fill their fins before gluing them in place and others fill their fins after.

Is one better than the other?
Definitely easier to sand the fins if you apply filler and sand before they are attached to the rest of the rocket.

Taken one more step, also easier to apply filler to fin stock and sand before you cut out the fins.

I just don't happen to like that the filler essentially "interferes" with adhesive penetrating the wood grain when assembling the fin to the BT. To me, first priority is a solid structural joint. If I were to pre-seal the fins (before ass'y) I would cover a 1/4 inch zone at the fin root with masking tape to keep that balsa/basswood/plywood clean, so the glue could still get a good grip later.
It is easier to fill the balsa grain before gluing the fins on the rocket body.
The more complex the fin arrangement, the more reason to fill the balsa beforehand.
Most kit directions have you glue the fins on, fillet the fins, then fill balsa and paint the model.

It's near to impossible to sand filler off near a fin root after you've made fillets on the fin / body tube joints. White Glue doesn't sand easily.

As far as strength of the joint goes:
If you fill with Elmer's Wood Filler, white glue will permeate the filler and wood when making fillets. The wood filler is water soluable.
If you fill with Sanding Sealer, the wood is SEALED! White Glue won't get into the wood.

Either way, if you fill the fins off the model, keep the filler off the root edge.
Thats why I just paper my fins with wood glue and printer paper. No sanding and filling and refilling. It adds strength and is lightweight.
I'm with DM1975. Papering the fins is the way to go unless your talking high power stuff.
annother vote for papering.

the only time I might fill a fin before attaching is if it will be hard to get to later, like on some of the TLP kits , fliskits Thunderbird, and a few others I can't think of right now. also they don't warp as easily if they are already attached
Ok I'm intrigued..............

What kind of paper?

How do you handle edges?

The fins I'm working on are a right angle with 3" sides.
I'm with DM1975. Papering the fins is the way to go unless your talking high power stuff.

Why exclude HPR? Granted you don't usually use balsa for HPR fins, but I've papered the fins on MPR and I'm sure they would have held up fine for G or H motors. Probably I motors too. At least the up part, it's the landing that becomes an issue.
Ok I'm intrigued..............

What kind of paper?

How do you handle edges?

The fins I'm working on are a right angle with 3" sides.

I use regular old printer paper for small stuff and card stock for bigger stuff. Just coat the fin with some wood glue and stick it on (do both sides so the fin doesn't warp) On the edges I either just give em a good coating of wood glue or I fold the paper down on them as well and glue the paper down. If I sand an airfoil I to the fins the I fold the card stock in half and lay the curved edge of the fin into the fold and glue it up kinda like a taco ;)

Almost all rockets with balsa get this treatment from me. I have even used paper over the fillets and launch lugs as well. Makes a nice, smooth transition.
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After the wood glue dries completely you can run a thin line of CA over the edges where the fin and paper meet. That can stiffen the edges of the paper, so they can be sanded to an airfoil if you like.
Another vote for printer paper.

Here's what I do:

Cut or pop out your fins and sand in whatever airfoil you want. I start by stack-sanding them to get them all the same size, then individually round the leading edges. For scale rockets I usually leave the trailing edge flat, since most scale rockets do that, but for sport rockets I usually airfoil the trailing edge.

Get a few sheets of printer paper. Lay a fin on the paper about 1/2 inch from the edge of the paper, usually with the trailing edge near the paper edge. Place the fin so it can be rotated over the LEADING EDGE toward the center of the paper. Make sure the paper will totally cover the fin (swept fins can be rather tricky because of the way they "fold over" at the leading edge. Once you're happy with the placement, lightly outline the fin in pencil, roll the fin over the leading edge, and gently outline it again. Sometimes you can get all the fins on one sheet of paper, sometimes a pair, sometimes you need a sheet per fin, depending on the size of the fin.

Spread an appropriate size dollop of white glue very thinly and evenly on the paper. The appropriate amount of glue depends on the fin size, naturally. Spread the glue very thin with your finger or an old credit card (the used up gift cards that accumulate on top of the gas pumps at Walmart work great, and they're FREE!) Place the fin in it's outline and press down firmly. Spread another dollop of glue on the other "outline" and once it's evenly spread and covered (be sure you actually go past the outline a little bit so the paper will be TOTALLY glued down), gently pull the fin and paper taut and fold the fin over the leading edge, and press it down firmly onto the glued paper.

Take a sharpie marker or other "fat" round object (I like Sharpies because the barrel is cylindrical and the tip is smooth round plastic) and then gently "burnish" the paper down, squeezing out all excess glue (there shouldn't be a LOT of excess glue, but SOME). Start from the leading edge of the fin and work the paper/glue toward the rear, from the center toward each end. This will ensure there are no wrinkles and the paper is on the fin TIGHT, and any excess glue is removed, since it only adds weight and isn't needed. Flip the fin over and repeat for the other side, from the leading edge to the trailing edge, and from the center to the tip and root edges.

If the fin is airfoiled, be sure you gently burnish the paper down to the tapered trailing edge on both sides, and burnish the paper down to itself just past the trailing edge.

Set the fins aside overnight to dry.

Once dry, trim the fins back to about 1/4 inch or less of paper sticking out from the edge of the fin with scissors. Get a SHARP hobby knife (new #11 blade works well) and gently 'shave' the paper off the edge of the fin, right up to the edge. OF course you only shave the paper off the root, tip, and trailing edges, NOT the leading edge. With a little careful work you can PRECISELY shave off ALL the excess paper and dried glue and leave the fin perfectly finished and ready for gluing onto the rocket.

If necessary, you can finish the edges VERY neatly with a piece of 220 grit sandpaper laid flat on your worktable. Stand the fin on the edge you want to finish, tilt it a TINY bit toward you and then gently draw the fin across the paper. Repeat as necessary. This will shave off any remaining 'paper hairs' or excess glue and leave the fin perfectly finished, ready to glue onto the rocket.

It takes MAYBE an extra 20-30 minutes to do the whole thing, and it GREATLY increases the strength of the balsa fins, and WILL NOT peel up from the leading edge from the slipstream ripping past, and totally eliminates sanding and filling the fins. White glue is best for the paper/wood joint like this and will not affect the penetration of wood glue into the fin when you glue it onto the rocket. If desired, the paper CAN be coated with thin CA after your done but it's not necessary.

Try it, you'll like it! OL JR :)
He nailed it. One addition: Make certain you place the fins on a flat surface and put some weight on them so they don't warp.
I've noticed that some fill their fins before gluing them in place and others fill their fins after.

Is one better than the other?

I don't think so. It all depends on what you've become accustomed to in your building and sanding technique.

Personally I almost always attach fins and most other parts before any filling or sanding is done. Because I do not trust glue or other materials applied to fillers it just makes me nervous prefinishing most fins and other details. Not Always but Generally; I prefer to glue and Fillet my fins before any sort of filling so I can smooth in shape these areas to as seamless an area as possible before applying most anything.
As far as filler go on body tubes, I don't bother anymore, preferring to use my Primers to fill out any minor seams and grain rather then fillers which tend to pop off or out at the worst times.
Some folks swear by Fill-n-Finish or whatever it's going by these days. Others use spot putty fillers, Green or White Squadron putty, etc. etc. If thats how ya want to spend our time that's great, I find I can get smoother, Cleaner, LIGHTER, finishing by skipping the fillers altogether and properly sanding the primers until they are almost gone. That said there are always times when a filler can be usefull on exceptionally deep body seams or grain, but for general building I personally believe most primers will do the job quickly and efficently.
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I agree. Most reputable rocket supply dealers sell tubes that are excellent. If you have purchased body tubes that have deep groves, however, you may be better off sealing, otherwise you will be applying lots of primer coats.
Roger that, Micro. The last few times, I haven't filled spirals with anything other than paint. I've had good results with both Kilz (ymmv) and Rustoleum Filler Primer.

Same goes for plywood fins; I just prime them after they're attached and sand smooth. For balsa, I usually use Elmer's wood filler stuff on both sides, then sand, coat again if necessary, and cut them out.