Tube prep before painting

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phoeg

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Hello all -

I'm a newbie to all this. returning to rocketry after about...38-40 years.

I am working on the classic "Big Bertha" and so far so good. After a lot of searching the forums I found plans for a simple fin alignment jig - a version of which I built this morning in under an hour from parts I had on hand. The next steps are to seal/sand the fins and tube, attach the fins, then paint.

After two days of searching the forums for various topics I've lost the bubble. Please forgive me that I couldn't find the answer in one of the other posts. Very simply: after I fill and smooth the tube spirals (carefully sanding so as not to sand through the--is it "glassine"?--finish on the tube) then prime and sand using very fine grades, should I sand through any primer (to the tube) where the fins will attach to allow good adhesion of the fins to the tube? I thought about running thin strips of painters tape along the line where the fins will attach so that all the filler and primer will go everywhere but there, but that's a random thought.

Thanks for your time, and again, I apologize for not being able to find a good answer elsewhere.

Paul
(Oklahoma City)
 
phoeg said:
Hello all -

I'm a newbie to all this. returning to rocketry after about...38-40 years.

I am working on the classic "Big Bertha" and so far so good. After a lot of searching the forums I found plans for a simple fin alignment jig - a version of which I built this morning in under an hour from parts I had on hand. The next steps are to seal/sand the fins and tube, attach the fins, then paint.

After two days of searching the forums for various topics I've lost the bubble. Please forgive me that I couldn't find the answer in one of the other posts. Very simply: after I fill and smooth the tube spirals (carefully sanding so as not to sand through the--is it "glassine"?--finish on the tube) then prime and sand using very fine grades, should I sand through any primer (to the tube) where the fins will attach to allow good adhesion of the fins to the tube? I thought about running thin strips of painters tape along the line where the fins will attach so that all the filler and primer will go everywhere but there, but that's a random thought.

Thanks for your time, and again, I apologize for not being able to find a good answer elsewhere.

Paul
(Oklahoma City)
Click to expand...

If you haven't yet attached your fins, then yes, by all means sand through the filler/primer and get down to the base cardboard tube. Most people actually build the rocket structurally then start the fill/finish work, but it can be done both ways. Good luck on your build!
 
Welcome to the forum. Big Bertha is a great rocket, I built one in 1968.

The only thing that should be done to the body tube before attaching the fins is a light sanding, and maybe filling tube spirals. But you should definitely not spray primer until after the fins are attached, otherwise the fins will easily break off. The fins need to be glued to the cardboard, not the paint.
 
I always fin the spirals before attaching fins or anything else.
I find they just get in the way.

I find bondo glazing and spot putty to be the best spiral filler.
The only trick is to give it enough time to dry.

Bones
 
Before gluing on the fins,
I fill the tube spirals with thinned CWF.
Primer follows. I sand the primer filler down more than some do.
The primer is then sanded down to the tube right at the fin glue lines.

X Cruiser 40.jpg

Everybody has their own way of doing it. There is no wrong answers here.
You just have to find what works for you.
If you primer the tube before gluing on the fins, sand off the primer at the gluing points.
Sand off the primer a little wider than the fin thickness they'll stay put after gluing.
 
Last edited:
Thank you, everyone for the feedback! It's much appreciated. One last dumb question: when sanding "down to the cardboard tube" do I sand through (or really just scuff/take the shine off) the glossy coating on the tube?

I ask because I don't (yet) know how that coating affects adhesion of glue, i.e. does the glue adhere just fine to that glossy coat or is it best to just scuff it a bit to give the glue something to "bite" into, as it were? Maybe it's a moot point because I'm lightly sanding the tube and any filled spirals already?


Thanks again.

Paul
 
Just a scuff should be fine. Honestly for low power rockets you don't even have to sand at all, unless perhaps if you know a lot of hard landings are in its future. Same for the launch lug(s). It is a good habit to get into though if/when you move up into mid power.

Some people enjoy "overbuilding" and will wipe, sand, double glue, epoxy filet, etc. Some might just use a line of white glue and call it a day. Don't sweat whether you're doing something the right or wrong way, just go with whatever feels right to you at the moment and enjoy yourself without the worry. There are pros and cons to every approach and so many times the answer is all "just depends" anyways. Personally, I try to use different building techniques on every rocket so I can gain all that "just depends" type of knowledge from first hand experience.
 
I'm a little late on this one, apparently. According to previous posts by Chris (hcmbanjo), wood filler doesn't cause any problems with adhesion, as the adhesive will soak right through it into the cardboard. PRIMER on the other had (and presumably sander/sealer) DOES interfer with bonding, essentially you will be gluing paint to paint.

I did a rocket this way before and did your tape idea (estimating the places of tube to tube contact and taping them off) and it worked really well, especially as I wanted to paint the tubes a different color than the fuselage. Build thread here.
https://www.rocketreviews.com/umm-me-uranus-explorer-2-scratch-by-tom-markel.html


)
 
Another trick is a line of pin holes through the body tube where you attach the fin. The line of glue over the pin holes wicks in and forms a better mechanical attachment. I would still sand through primer down to paper as well.
 
When I lightly fiberglass (.75oz glass and finishing epoxy) an LPR bird, both BT & surface mount fins, I'll drill 3 or so holes along the body tube fin line for some additional mechanical attachment. Epoxy the root edge and fillet. This method would probably work well with regular cardboard BT & unfinished balsa fins... the only issue would be that the glue and joint will be stronger than your tube or fin resulting in a broken fin failure as opposed to just snapping a fin off on a hard landing. A fin that pops off is a much easier fix.

Jerome
 
Thanks again for the advice. I did read elsewhere about the pinholes and had planned to do that. I used the Elmer's carpenters wood filler (thinned with water) an applied to the spirals with a small model brush. I then lightly sanded with 320 & 400 grade; came out nice and smooth! Success! :D

I have filler on the fins drying now on one side and when that's dry I'll sand and do the other side. I'm amazed at how much smoother balsa gets with filler. Wow, the things I'm learning!

After they're filled and smoothed I'll glue the fins before priming. I can't imagine that I'll need to use much primer. I try to make everything as smooth as possible first (something I learned in auto body years ago :) then I'll lightly sand that before painting.

Thanks again, everyone!
 

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