Tissue and Dope

Tissue and Dope is a tested and true way to seal balsa model boat hulls while adding strength. This how-to gives step-by-step instruction for sealing balsa with rice paper and cellulose lacquer.

Tissue and Dope used to be so common in model boat building that nobody bothered to document the process. At least I have never come across a good source that explains how to do it.

PT boat hull sealed with the "tissue-and-dope" process.

I remember reading the instructions for a Guillow’s balsa airplane kit many years ago, but the application to a balsa frame versus solid balsa is quite different. Here I’ll focus solely on the purpose of sealing model boat hulls.

Many balsa models are built as working models, meaning they are built to be sailed on the water. Because balsa is so porous, it will start to leak almost instantly as it is placed in the pond, bath tub or whatever. In addition, a raw wood hull, balsa or otherwise, will not handle the cycle of wetting and drying many times before deteriorating.

Balsa is incredibly soft and needs a hard shell to protect from any kind of handling. Because the dope is absorbed into the balsa, the hard shell that forms is integrated into the wood as well as adding a coat on top of it.

The function of the dope is pretty clear. The tissue contributes the following benefits:

First, the tissue adds strength to the shell structure just as fiberglass does in a resin or epoxy matrix. This may be the most important reason. Lacquer all by itself has a tendency to crack or craze if subjected to dings and abuse.

Second, it helps the dope stay on the surface without getting absorbed deep into the balsa, thus adding weight.

Third, if we were to just brush on and sand between coats (just like in a traditional wood sealing paint schedule), we’d all be old and gray before we got rid if the raised grain ("peach-fuzz") and started building a true coat.

The Tissue and Dope process address all these issues.

What is Tissue?

Tissue in this context has nothing to do with Kleenex or anything else you’d use to blow your nose or wipe a tabletop, or bottom.

Instead, this is the same paper sometimes referred to as "rice paper" or "wrapping tissue". It’s a lot more crinkly and noisy than the Kleenex stuff.

Tissue is becoming more and more difficult to find in hobby stores. Buy no despair, it’s readily available in craft stores and many art supplies stores. Here in the US it is commonly used for gift wrapping and many craft projects.

If you’re thinking of doing a lot of models, and want to stock up, consider getting a bulk pack from Uline or Xpedx. What you’re looking for is "gift grade tissue paper". They sell another product as "tissue paper" for wrapping breakable things, but it’s more like news print. While this paper may still work, it’s completely different and I’ve never tried it.

Most of the tissue paper you’ll see comes in all kinds of bright colors. This may seem completely frivolous and unnecessary. However, there is a potential benefit here. As you lay alternating layers in contrasting colors you may be able to tell when you’ve sanded through a layer by watching the color of the dust. I say "potential" because I have not yet perfected the technique.

The colored tissue paper comes in two varieties. One made with waterproof dye and the other will bleed if wet with water. Neither will bleed with dope, so either will work fine for our purposes.

What is Dope in Tissue-and-Dope?

Dope, varnish and shellac are all the same, right?

Wrong!

While varnish and shellac may have some uses in model boat and ship building, I’d stick with "dope" for the tissue-and-dope process. It cures a lot harder than either of the other products, and is a lot more durable.

The proper name for dope is Nitrocellulose Lacquer. It has been around since the 1920 in industrial and commercial form. It is best dissolved with lacquer thinner. Both can be bought in many hobby stores, but chances are you’ll get more for your money if you go to a home improvement, hardware or paint store.

Lacquer also comes in spray cans ("rattle cans") which is nice for those final streak-free coats. There is also "Brushing Lacquer" vs. just "Lacquer". The former is supposed to dry slower to allow for more time to apply.

The biggest benefit with Lacquer over any other clear coat is that each layer fuses together with the previous, forming a solid entity. This makes for a more durable and strong coat than just about any other comparable material.

A word of caution: Lacquer and Lacquer Thinner contain volatile chemicals and should only be used away from open flames and with ample ventilation. I only use it outdoors.

The Tissue and Dope Process

Here is a quick step-by-step how-to of the Tissue and Dope process:

1. Dilute: one part Lacquer to two parts Thinner in a glass jar and mix well. I like to use salsa jars, because they are wide at the mouth. This leaves plenty of room for dipping the brush!

Here are some of the things you'll need: Lacquer (AKA "dope"), Lacquer Thinner, a mixing jar, tissue paper, scissors, a brush. Also useful are: brand new single-edge razor blade, disposable gloves (optional).

2. Cut out creases and hard wrinkles from the paper. It's virtually impossible to prevent bubbles and voids to form unless these parts are cut out and eliminated.

Don't use scissors with serrated blades unless you love sanding.

3. Cut the tissue paper into manageable size pieces. It's good to have a selection of sizes at hand when you are getting into it. For this project the smallest pieces were about 1/2 x 1 inch (about 13 x 25 mm) up to 5 x 3 inches (about 130 x 75 mm). Make sure you cut more paper than you are going to need.

Sand the hull smooth before beginning the process. Don't fool yourself, imperfections will show.

4. Make sure the hull has been sanded smooth and is free from sanding dust. This is not the place to cut corners, but not to go crazy either. Imperfections will either show as indentations or voids and bubbles. Indentations where the tissue has adhered to the material underneath is OK - we can use fillers later. Bubbles and voids are problems that need to be corrected before proceeding.

Start out by covering the most difficult areas first. The paper will not take a compound curve well without wrinkling, so cover the spine and knuckle lines with small pieces (like scale on a fish).

5. Start covering the hull. I like to begin at the sharpest corners first: the keel, then the chine (where side and bottom panels meet) and between transom ("stern panel") and adjacent panels. By using narrow strips across the corners the tissue has a better chance of staying where I'm putting it. If the strip is too wide, a void will form right at the corner as the tissue is pulling away from the hull.

First coat done. The observant will notice I didn't cover the chine line with "scales" the first time around. I learned the hard way...

6. It's tempting to do a second layer of tissue and dope "wet-on-wet" to save time. Don't do it. Somehow doing so creates voids and separations between the first layer and the balsa. Be patient and only do one layer at a time.

A close-up of the hull after the first layer, and just barely into the second (notice "fish scales" along keel and chine). The bare wood spots is where voids have been eliminated.

7. Once a layer of tissue and dope has dried for 24 hours it's time to sand. I used 220 grit sandpaper - it's on the fine side, but I'm paranoid of going at it too hard - the tissue is thin and very easy to sand through. Voids are hard to see, but they look like blisters. As you push lightly you notice the tissue is not attached to the underlying hull and "gives". It's almost futile to sand them out. The best way is to carefully puncture the "blister" with a fresh new razor blade and carefully trim the edge from the center outward. Once the void is gone, carefully sand the edge smooth and prepare for another Tissue and Dope layer.

Second layer done on the keel, chine and bottom panels.

8. Rinse and repeat...as they say.

First layer of tissue and dope on the deck done.

9. At some point it's time to quit Tissue-and-Dope and switch to Dope-and-Sanding to start building up coats of just lacquer. I figured two layers of tissue ought to be enough on average. Keep adding coats of lacquer and sand with finer and finer grit sandpaper until the hull is smooth.

The PT boat hull sealed with the "tissue-and-dope" is almost done. More Lacquer and sanding is needed.

Continue to RC PT 109 Part Six: Building the PT 109 Deck Houses

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