Friday, November 24, 2006

AV - Varnish

> > Do you varnish the inside of all the drilled holes and underneath all fittings before assembling wings? <<


Because a hole exposes the end-grain of the wood it usually receives extra attention, such as blocking the back-side of the hole with your finger and FILLING the hole with diluted varnish, poured from a small can, etc. Wait a few moments then position the can under the hole and remove your finger. (Proper orientation assumed.)

You won't appreciate the need for this until you've removed the fittings from some older wooden structures. Or rather, tried to remove them :-)

Even with cadmium plated AN hardware you'll often find bolts corroded solidly into the wood, fittings deeply etched with rust on their back-side and so forth. The VP's landing gear attachment bolts are especially prone to corrosion due to their location and the depth of wood.

If you want to add a sealant to the shank of the bolt you may find paraffin (white mineral wax) to be a better choice than vanish.

>>Thanks for the reply. I was contemplating using Q-tips to varnish the holes......certainly like your method better.<<

Before Q-tips there were patches.

Traditionally, a 'patch' was piece of cotton fabric about the size of a silver dollar. You made them out of scrap left over from a covering job, or cut them out of tape. For repair work you had doped patches and plain. Fabric-covered aircraft that actually worked for their living were always getting holes poked in them. The typical hole resulted in an L-shaped tear. Small tears, you'd use a curved needle to take a couple of baseball stitches to hold the tear closed then apply a doped patch. (The idea here is that the dope was the same color as the airplane; otherwise you used a clear-doped patch.)

Nothing really new. In Vietnam we used aluminum beer cans and a smear of RTV. (By the Vietnam era most fabric covered control surfaces were Razorback -- fiberglas, rather than cotton.)


Point is, to varnish a drilling in a wooden structure, if you couldn't flood it with dilute varnish you poked a piece of safety wire through the hole, made a little hook on the end to catch a varnish-soaked patch. Then you used a soda straw, piece of tubing or a pump-can oiler to flood those outta-postion holes, the patch being pulled partly into the hole to plug it... and finally through the hole to 'paint' it.

Some guys used a rib-stitch needle and a triangle of tape; poked a corner of the tape through the eye of the needle, sorta twirled it to make the plug.

And 'tape' means a roll of cotton fabric, two to four inches wide with pinked edges. And pinked edges means.... (this could go on all night)


Kind of an interesting point in all of this, in that while most assembly and re-covering manuals talk about sealing holes in wood, I can't recall any that told you how to do so. The methods I've described above I learned from my dad, an old time A&P, or from other mechanics.

Also note that all the stuff I've mentioned -- safety wire, fabric tape, patches and so on -- is stuff that would normally be available & near at hand if you were working on airplanes. Working in your garage, covering with dacron, if Q-tips are all you got, then usem.

The important point is to provide a good seal inside every hole through wood. Aircraft wood is twelve to fifteen percent water by weight. Softwoods, such spruce, pine, hemlock or fir... the stuff commonly used in aircraft to absorb moisture and does a good job of transporting it from one end of a stick to another, which is why stored wood usually gets its end-grain sealed with wax, tar or paint.

After the wood is used to build something, the last step in the fabrication is to seal the whole surface of the wood. Once you've sealed the wood with varnish or whatever, its moisture content remains fairly stable and if protected from sunlight, it sort of goes to sleep -- it stops aging, or at least, slows down to the point where the process is not apparent to humans. Periodically, such as when we replace the fabric, we re-new the seal of the wood. This isn't unique to airplanes, it is the natural order of things that applies to anything made of wood.

> I think 50/50 is too thin. 90/10 is more like it. The very experienced fellow who painted and/or varnished everything in and on our house told me this.<<

Houses aren't airplanes :-) Experience derived from house painting or furnature building is of little use when it comes to protecting the structure of a wooden airplane.

Airplanes are largely built of softwood. The first coat of varnish should in fact be little more than thinner. The objective is to seal the wood at the microscopic level, which thinned varnish does perfectly well... if you thin it enough. For spruce, fir and pine a first coat of only 25% varnish to 75% thinner is not unusual (ie, ratio of 1:3)

The first coat is allowed to dry until #120 paper produces only a dry white powder with no clogging at all. The sealed surface is then sanded lightly. 'Scuffed' was the old-fashioned term; some times you heard it described as 'dulled' but either definition leads to misinterpretation unless you've seen the procedure being done. It is basically a light but complete sanding with fairly fine paper, after which the surface is wiped down with a clean tack-rag, frequently turned. The finish coat is usually 75% varnish thinned with 25% thinner. (ie, ratio of 3:1) Nowadays I suppose everyone uses White Mineral Spirts as thinner. When using real spar varnish we used turpentine.

The above procedure is valid for the interior structure of wooden aircraft. For the exterior -- wooden struts, gear-legs and tail skegs, the second coat was given an additional sanding; the final coat was full strength varnish, properly laid-on. Varnished exterior surfaces were frequently inspected and renewed as required. Interior structures were sanded & renewed with each re-cover. (Ed. Note: 'Full Strength' might still mean some degree of dilution with thinner, especially if the varnish were old. The reason here has to do with application rather than penetration, in that the varnish must be thin enough to flow-on in a smooth coat.)

A point many overlook is that with airplanes, the finish is supposed to weigh as little as possible. This dictates methods and procedures that are never used with furniture, gun stocks, marine bright-work and so forth, each of which differs from the others to some degree.

As for application of the final varnish coat with a spray gun, while this is commonly done when refinishing props, struts and large panels of fabric(*) or plywood it is seldom used for the interior structure of wings due to the large number of edges, nooks & crannies, for which a brush generally goes a better job.


(* - At one time varnish was a common finish for cotton & linen fabric.)

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