Best way to prep plywood fins for priming?

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Bill S

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I'm just starting out with using 1/8" plywood for fins, and am wondering what is an easy and lightweight way to fill in any grain before priming? Normally with balsa fins, I use Bondo Glazing putty, sand, prime, fix any defects, reprime, etc. Then sand with some 800 grit right before painting.

Also, are the edges handled any differently than with balsa? I normally use the same Bondo glazing putty to fill in the grain and sand.
 
I use GoodFilla to fill the grain and sand. Dries very quickly. Only takes about 30 minutes to be ready to sand. Same technique just different filler. And the edges of my fins are airfoiled. Not a lot of wood to fill.
 
. Then sand with some 800 grit right before painting.

Also, are the edges handled any differently than with balsa? I normally use the same Bondo glazing putty to fill in the grain and sand.
800 doesn't leave the surface with anything for the paint to really grip to. I never go beyond 320 grit before painting, same for the edges.
 
I had mixed to poor results using various high build and zinc primers.

At suggestion from a friend, I put several coats of shellac on the plywood fins and balsa nose cone. Sanded with 320 between coats. Turned out great!!

Shellac lays down flat, dries quickly, and is sandable right away. Doesn't require a good brush. Smells nice too, takes me back to 7th grade wood shop...
 
I had mixed to poor results using various high build and zinc primers.

At suggestion from a friend, I put several coats of shellac on the plywood fins and balsa nose cone. Sanded with 320 between coats. Turned out great!!

Shellac lays down flat, dries quickly, and is sandable right away. Doesn't require a good brush. Smells nice too, takes me back to 7th grade wood shop...
Do you use the famous Bulls Eye brand?

Bulls Eye® Shellac is an alcohol-based solution of pure lac, a natural resin secreted by tiny insects on certain trees, found mainly in Southeast Asia and India. After it is harvested the dark, reddish-brown resin is crushed into granules, rinsed to remove the natural dye and then dried.
 
So maybe some wood sealer from Lowes/Home Depot and then sand with fine sandpaper then?
 
I sand the sides initially w/220 or thereabouts, do any airfoiling as desired, then just as I would with balsa fins; thinned CWF followed by another round of sanding.

Before CWF I mask off that portion of the fin which will be covered up by a fillet.
 
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My plywood fin process:

-Smooth all over with 220 if wood is fuzzy.
-Airfoil edges with 100
-Smooth edges with 220
-Coat with sanding sealer
-Smooth with 220
-Repeat previous 2 steps as needed (usually only takes me 1-2 coats to be satisfied, but I have done 3 for particularly rough wood)
-Smooth with 400
-Install fin on rocket (proceed to finish rocket and prime/paint the fin along with the rest of the rocket)

Lots of ways to do it, but that's what has been working for me.
 
800 doesn't leave the surface with anything for the paint to really grip to. I never go beyond 320 grit before painting, same for the edges.
I've heard this before and while I don't disbelieve it, I definitely don't understand it. Many aspects of painting remain a mystery to me.

First of all, I've had good enough luck with 800-sanded primer. I just painted a rocket and used 800 grit to very lightly dry-sand the primer, just to remove the rough bits (which it did, very nicely). My main reason for using 800 was that I didn't want to remove too much primer, since I didn't have that heavy of a coat.

But what confuses me even further is: if paint won't adhere well to an overly smooth base, why does it work to apply a second gloss color over the base gloss color? I don't recall ever having paint adherence problems there, and I've never heard anyone recommend that a gloss base coat should be "roughened up" before applying a next color (not that it's practical anyway, for most rocket paint jobs).
 
But what confuses me even further is: if paint won't adhere well to an overly smooth base, why does it work to apply a second gloss color over the base gloss color? I don't recall ever having paint adherence problems there, and I've never heard anyone recommend that a gloss base coat should be "roughened up" before applying a next color (not that it's practical anyway, for most rocket paint jobs).

The solvent “carrier” - typically acetone based but not always - reacts with the the cured surface level of the base coat, sort of melts it a bit, so the paint adheres. It’s the same process that can cause the paint to wrinkle up when the top coat solvent reacts too much, say with a base coat that hasn’t fully cured. There are times when the base coat does need roughed up, or at least scuffed up enough for the solvent component of the top coat paint to react with the base coat - some epoxy paints cure hard enough (producing a surface that’s so hard, smooth and neutral) that the solvent has nothing to work with - think of the surface as a ‘lock’ and the top coat paint as a ‘key’.
 
So then why does primer need to "have tooth"? Does the same solvent reaction not apply?
All paint needs some amount of “tooth” to allow the physical side of the adherence to happen - though with latex paint the reaction is pretty much all physical as the paint self-levels, which is why with latex you need to lay down each coat without overworking it - primer is designed to provide a substrate that paint can grab, both mechanically and chemically. Paint is pretty nifty stuff and a bit more complex than we tend to think it is - I’m not an expert of any kind, just a dilettante with enough of a grasp of chemistry to enjoy going down the rabbit hole when reading about how it’s supposed to work. And enough years of experience using (and screwing up) multiple types of paint on multiple surfaces to know what works for me - which, if you ever see anything I’ve painted in person, you’d understand that “works for me” is a finish that either looks good enough from 5 feet away or looks good enough that my wife doesn’t ask me to repaint it 😆
 
To clarify: when you say epoxy resin, that’s the resin without mixing it with hardener (parts A & B)? Will the resin dry on its own?

I recently had a plywood fin break on landing, probably due to a defect in how the plywood was manufactured. After reattaching the fin with CA, my repair will involve a fiberglass cloth skin on each side of the fin, so no filler needed. Just lightly sand the epoxied skin, prime and paint.
 
To clarify: when you say epoxy resin, that’s the resin without mixing it with hardener (parts A & B)? Will the resin dry on its own?

It is both parts. It MUST be mixed. That is just the common term for the system.
 
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So then why does primer need to "have tooth"? Does the same solvent reaction not apply?

Your solvent reaction question reminds me of a conversation I had a couple decades ago with Grant Victor, the fella that painted my old hot rod Fords. He said it's always a good idea to use a paint system from the same supplier. That way each different product (primer, filler, base coats, top coats, clear coats) are all designed to be compatible with each other.

I also remember him saying the initial application of primer required a tooth, and as I recall they were using 80 grit on the orbital sanders to get that. Sandblasting is also great, it leaves a tooth on the steel.

Whatever I'm painting, from steel hot rod parts to cardboard rockets, I tend to try to rough up the surfaces until after the initial coats of primer... based on Grant's advice all those years ago. Getting the surface smooth happens after the primer is applied.
 
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Do you use the famous Bulls Eye brand?

Bulls Eye® Shellac is an alcohol-based solution of pure lac, a natural resin secreted by tiny insects on certain trees, found mainly in Southeast Asia and India. After it is harvested the dark, reddish-brown resin is crushed into granules, rinsed to remove the natural dye and then dried.

Yes, I think it is Bulls Eye. I'll confirm later, can is at my GF's house.
 
Sand the fins as needed. I typically go up to 320.

Next I use a good quality wood filler. Currently a big fan of the DAP brand. One or two thin coats applied using a putty knife then sand. Normally knock off the high spots with 120 grit, then 240 and 320. Next it's two or three coats of sanding sealer. I like Min-Wax brand but there are other that work just fine. Let that dry over night then sand. Again knock off the high spots with 120 grit, then I typically go to 320 before doing my final with either 400 or 600 wet.

A couple coats of SEM brand filler primer, sand with 400 then 600 and it's off to paint.
 
With plywood, I've gone to using sandable primer (Dupli-Color is my favorite brand, from auto store), then sand it with 180-220 grit paper and repeat. Gives good results. Do the same for balsa wood, except I do one coat of sanding sealer (or thin CA) and sanded before I start with the sandable primer.
 
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