Each coat of filler should be sanded down until you start to touch balsa, therefore added mass due to any filler should be negligible.

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I love sanding.

Well OK yeah sure - most of us know the technical correct way of applying a filler. I suppose then there is no difference if we used something lead-based as a wood grain filler.

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-Wolfram v. Kiparski
NAR 28643 - TRA 15520
MTMA Section #606 President

You can still get the equivalent of Aerogloss dope-based Balsa Fillercoat and Sanding Sealer from Brodak in cans as small as 4oz (1/4pt) and as large as Quarts. Their "Primer" is the same as Balsa Fillercoat, but is White, not light grey/light tan. Their Sanding Sealer is the same as Aerogloss, and the same consistency, unlike Sig's which is far too thick.
I still have loads of Aerogloss, but often use Brodak Primer (Balsa fillercoat), 3 coats, sanding between coats 2 and 3, then 2 to 3 coats Sanding Sealer on top, sanding between each coat.
It competely fills the grain ahd provides a glass-smooth finish for topcoating with ANY topcoat including Dope, Lacquer, Enamel, Epoxy, or catalyzed Urethane.
NOTHING beats it for filling wood parts. the weight is FAR lower than using spray-bomb primers too.
I have 2 LHS's that carry Brodak products, but you can order from them as well.
I wouldn't use a water-based filler to re-finish an old beat-up Alpha. That stuff is CRAPPPPPPP.

__________________
When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !

You asked and I provided a factual answer without putting you down so I don't know why you are upset. However, I apologize if I did offend you. With correct filler application and sanding, the density difference of the filler should be within the margin of error that would include having test fins with exactly the same size/density, and the ability to sand each fin to the exact same tolerances. Filler choice should be made based on which filler is easiest for a person to obtain and get a good finish, along with considering fumes, cleanup, etc.

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I love sanding.

I love dope products, been using them since the 70s. Over the years Aerogloss, my favorite has dwindled down to nothing. I have bought some off of ebay but it pretty much was worthless. Even Sig has down sized their colors and no longer carry their spray dope. Brodak is my supplier for color dope but have not tried their sealer yet. I did order a small jar of Sig Sealer with my last Sig model order and I was not impressed. I thinned it to the consistency of Aerogloss dope and it brushed on fine, but it was much harder to sand than Aerogloss. I use 3 to 4 coats of Aerogloss balsa sealer with no filler and my fins come out smooth and grain free with little effort. I will give Brodak a shot, think I need to order some Silkspan anyway.

You'll like the Brodak stuff.
The ONLY difference is that I add about a teaspoon of thinner to a 4oz can.
I do the same to Aerogloss as well as I like it a bit thinner than 'stock'.
It is NOTHING like the Sig stuff that has to be thinned almost 50/50.

__________________
When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !

I'm hoping somebody sees and still responds to this old thread...

I know I stared this thread about a year and a half ago, but I'm just now finally getting around to actually finishing my first build using the primer only method. I need some further help regarding technique. I must be doing something wrong because so far, I don't see how doing primer only can possibly be easier than the old dope method. But I've read a lot of threads about it where people say it's so much easier, so I have to be doing something wrong.

A bit of background. Back when I built rockets as a kit, several coats of Aerogloss with sanding in between is what I was used to. Then I just topcoated after the final dope sanding.

Also, I'm not nearly the perfectionist where I worry about filling in spirals in body tubes. All I'm really concerned with is filling/finishing the balsa parts so the grain isn't visible in the final topcoat of paint.

The primer I used was the stuff I mentioned back in my post 15 of this thread, Rust-oleum Painters Touch 2X Ultra Cover primer. Here's a link back to that post.
http://www.oldrocketforum.com/showp...70&postcount=15

The other factor is that the kit I'm finishing is the Mega Mosquito, which has very large balsa fins, so a lot of balsa surface to seal and finish.

So why do I think I'm doing it wrong? I saw in several posts where we should be sanding most of the primer back off. I've been sanding for over an hour and I'm no where near getting the primer mostly off. I finally got it to a point where I think I'm ready for the next primer coat, and I have a few exposed areas of balsa on the fins, but not much. I don't ever remember sanding that much on Areogloss.

My first thought, did I put too much primer on for my first coat? I did spray until the balsa was pretty well coated. Is it supposed to just be very light coats?

What grit sandpaper to people use when sanding the primer coats? I started with 220 but wasn't getting anywhere so switched to 100 (it's what I have on hand). Then when 100 finally got me to a point I thought was good, I went back over it with 220 again to smooth it out. When I finish the last primer coat, I was going to sand with maybe 400 grit for the final sanding before top coat.

Should I use a block for sanding the primer, or just free-hand it? I always use the block sander when I'm sanding the raw balsa before assembly or before the first primer coat. But the block sander didn't seem to do an even job on these large fins. As flat as I tried to hold the block, it would sand more on the outside edges of the fin and not get much in the middle of the fin. I ended up having to free-hand it to get the middle.

Also, the sandpaper gums up a bit and gets little spots where sanded paint builds up in the sandpaper, which then starts gouging the very surface I'm trying to sand. This happened with both the sanding block and free-hand, and both with the 100 and 220 grits. Is this normal? It's like I have to use a clean piece of sandpaper on each fin, which would eat through a lot of sandpaper. (I could clean the spots from the sandpaper with my fingernail, but that was a lot of work in that it needed to be done constantly, so I finally gave up on it.)

I also realize I need to wear gloves when sanding the primer. The primer dust is embedding itself in the palm print lines of my hand just as if I had gotten wet paint on my hand.

Thanks much for any further tips on the primer only method.

Your building/finishing goals sound like my building style, you can search my builds and yes spirals do show. I also did a write up on primer only and that method worked but was more work than what it is worth. I don’t know why Estes now suggest the primer only method for filling balsa.....and two coats I have tried a lot of methods....Primer, FNF, papering, Deft wood sealer and balsa sealer. At the end of the day balsa sealer produced the best results with least amount of work. Chris on the Model Rocket Building blog has a great technique using FNF combined with primer before he assembles his rockets. Unfortunately this does not work for me since I build vintage rockets and enjoy the look of an old rocket assembled with naked cardboard and balsa.

If you're going to go "primer only," I would at LEAST use some kind of sealer on the fins, to minimize soaking-in of the paint.

Thinned white glue, allowed to THOROUGHLY dry, works just fine. It raises up the grain of the balsa, and you do need to sand that down, but the result will be a less permeable surface.

Again: Let the sealer dry really well. A week or more in a warm dry place. This is to prevent moisture from getting trapped beneath the paint and potentially bubbling up.

I've been using Kilz primer, which is sloppy by goes on really thick and sands off well.

If I were painting a plastic part with fine detail I'd use something else; a Krylon primer of some sort, or maybe a plastic model primer from Testor's, even.

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NAR #27085 - Oregon Rocketry - SAM

I have been involved in rocketry for over forty years and have tried numerous new-age finishing systems. I always go back to the tried-and-true old-fashioned organic-solvent based dope balsa fillercoat followed by sanding sealer for finishing wood parts. It is the lightest and works best BY FAR. I shoot lacquer based primer over the top of that. Trying to use primer alone SUCKS.

__________________
When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !