Brodhead Pietenpol Association

Evening folks, had a lot of time to work on the plane in the past year. Having said that I am at the point that I am putting the center section together and had a thought that stopped me from proceeding on the day I was planning to glue the ribs onto it. Even though I painted the metal brackets with the Gucci zinc chromate primer and paint I thought that having metal against wood would cause the metal to rust faster. Then I thought I could put varnish on it to help seal the wood but thought that might be overkill. Any thoughts on what would be best or should I just get back downstairs and get back to gluing.

Sean

all your wood (including in any holes drilled) should be varnished to protect the wood.

Yep, what Earl said. Varnish wood before placing metal fittings against it. Ken

Gents, thanks for the replies. i guess my next question(since i assumed i would be doing this much later and had time to research it) is are all varnishes(i.e. something i find at a big box store) the same? If so i will head to the BORG tomorrow.

Sean

The woodworkers in this group can provide you with the most sage advice on which varnish would be best in this application, and there are some real pros around here. What I have used on my airplane (which I did not build; only have had to varnish additions, repairs, and miscellaneous woodwork on it)- has been Minwax "Helmsman" oil-based urethane spar varnish, clear satin finish. My main reason for using it is availability... I can see our Lowe's store from my hangar, across the airport at about 090 degrees magnetic ;o) I like the oil-based product better than water-based, although it requires mineral spirits for cleanup rather than just water.

Oscar Zuniga
Medford, OR
Air Camper NX41CC, A75 power

Sean,

I used the Helmsman Spar Varnish, as well. The issue with varnish is not really which type will work. I suspect that any decent varnish will do the job of protecting the wood. The issue is what you will eventually use for your covering system.

If you later opt to use the Stits Polyfiber sytem, then any place where the fabric will touch wood needs to be varnished using a two part epoxy varnish. The solvent used in the Stits Polyfiber system will lift the one-part urethane varnish and make a mess of things -- in every sense of that term. If you later opt to cover with the Stewart system, the one-part urethane varnish is just fine.

One of my airplanes has the Stits system; the other has the Stewart system. Both work. I will only ever use Stewart in the future. The solvent at the heart of the Stits system is MEK, which is nasty stuff -- though in fairness some people like the smell, especially once they begin to get light-headed and slap-happy.

The Stewart system is solids in water suspension. I covered all of the smaller pieces of my Piet in my basement one winter using Stewart. No smell; no fumes; no drama.

So the question of varnish is really a question about the future covering system.

Cheers, Ken

Ken; smell and sensitivity are BIG factors to consider as we build our airplanes and engines, and some people are extremely put off due to one or both of those. My wife is very sensitive to odors and although that was not my primary consideration when I elected to use Aeropoxy to construct my little Flying Squirrel composite plane in our garage, she appreciated the very low odor from that product and never complained about it during the 5-6 years that I worked in our garage rather than at the hangar. Paint, varnish, solvents, gasoline, anything like that- she will give me the stink-eye and send me to some distant place to mess with that stuff. I, on the other hand, like to smell the materials I'm working with and I enjoy being in the hangar while working with Poly-Fiber materials. Then again, I know I'm an oddball because I also like the smell of 2-stroke when I run the chainsaw, and the smell of a big radial engine starting up, and things like that.

-Oscar

Oscar and Ken, thanks for the additional info. I had not even thought about the covering and any associated interdependencies with the varnish. Would I be correct in assuming the difference in the price between the two are negligible WRT the price of the entire build. I would lean toward the Stewart system if there are less VOCs but the wallet gets a big vote as well.

If you are going to use a water based covering such as Stewarts System then spar varnish works well and is what I'm using. Woodworking is my hobby and the preference for spar varnish in this application is that it doesn't dry as hard as other varnishes. That means that as temperature and humidity changes it is more elastic and less likely to develop micro fissures and cracks. Those cracks lead to water penetration and wood rot which you are trying to avoid by applying the varnish in the first place. This is one of the reasons why spar varnish is usually recommended for outside protection and not inside finishes. Marine grade varnishes have more protection from the sun but will cost considerably more and since you are covering the wood with fabric not a great benefit in this application.
I wouldn't use spar varnish on the inside of the cockpit or your seat. As it doesn't dry as hard it can be softer on a hot day and you may find your body or other objects print or stick to it. This is another reason spar varnish isn't preferred on indoor finishes. I personally would prefer marine varnish in the cockpit application. You can also finish the marine varnish to a really nice gloss if you want to. For furniture in the house I don't use either of these because I'm not worried about the sun and get a quality finish from other products made for the application at hand.
If you're going to use a covering that requires petroleum product adhesives then don't use spar varnish or other oil based varnish. The petroleum products will dissolve the varnish.
As if that's not enough; In EAA Aircraft Building Techniques by Corby Starlet, the author references treating wood with a preservative to prevent rot before applying varnish. I also have the four Tony Bingelis books and I have not found a reference regarding wood preservative. Besides my build, I am helping to restore a Travel Air at Liberty University and they are going straight to varnish.

Sean, all of the fabric products are exactly the same whether using Stits Polyfiber or Stewarts. The difference is entirely in the various "wet" products. The sticker prices are roughly comparable between the two. Stewart has an edge in overall cost as none of the items is considered hazardous material. Consequently shipping is generally cheaper and faster for the Stewart products. Best of luck with this, Ken

p.s. Jeffrey, I haven't noticed the spar varnish in the cockpit areas getting soft. That great big parasol-type wing tends to cast a lot of shade over the cockpits, so maybe that has helped keep things from going gooey.

Great Conversation here and I've heard good stuff about spar varnish and it's compatibility with the Stewart system (will work fine).

My question is this, if I've already got one coat down with a two part epoxy varnish and want to switch to the Stewart system will there be a problem laying a coat of spar varnish over the two part? My experience with boats is that there is no problem, but then again if the varnish on a boat peels off it still floats, layers separate between the wood n fabric on the flying machine and she might decide she's done flying. Any advice is welcome.

Erik, I don't know the answer to the question of whether spar varnish can be put on top of the two-party epoxy varnish. But it isn't necessary to do that. There's no problem with applying the Stewart system atop the two-part varnish. That will work just fine.

The problem is only if you are using the Polyfiber system atop the spar varnish. The base liquid in all the Poly system coatings is MEK. That will lift the spar varnish off the wood. Hence the need to use the two part varnish if going with Polyfiber. By contrast, Stewart system is solids in water suspension -- basically a type of contact cement. It will stick to anything without adverse consequences.

In fact, the Stewart STC (if you are doing this on a certificated aircraft) permits its application on top of Poly. The Polyfiber STC, by contrast, does not permit it to be used to repair any other type of existing system. I'm not criticizing Polyfiber. It's a great system. It works when the directions are followed, which requires surfaces where MEK can do its thing.

Cheers, Ken