Changing the patches on lower wings of the Caudron to a modern patching material

The patching of the holes on the lower wings of the Caudron C.59 CA-50), the advanced trainer from the 1920’s, was finished in 2021. More than a hundred holes of various sizes could be found on both wings. To patch the holes on the lower wings we had used first cotton and then linen fabric.  We had sewn together the tattered edges of big holes before gluing the patches on them. The patches had been fringed on their edges in the manner of the 1920’s, and they were glued on the holes using nitrocellulose lacquer, i.e. the tightening lacquer for the covering fabric.

In the autumn of 2022 we started the changing of the fabric patches on the lower wings with the Finnish Aviation Museum’s staff supervising the work. The reason for this was that the larger patches of fabric, and especially the linen patches, hadn’t properly tightened. There were also problems with those patches, where the edges of the holes had been sewn together. In these places the stitches had remained lumpy under the patches in a disturbing manner, probably because the fabric hadn’t tightened enough on the stitches. Patches on the wings have to be smooth at least for aerodynamic reasons.

The Museum decided to have the inadequately tightened linen patches and the fabric patches containing stitches to be changed to a new material. As the new material the modern heat-tightening Oratex- material, resembling the 1920s style, was chosen. Patches made of thin Oratex-material, painted in the patinated greyish green hue, produced inconspicuous and at the same time aerodynamically extremely smooth patches, compared with the fabric ones made with tightening lacquer.

To replace the patching material the fabric patches were loosened using thinner for nitro cellulose lacquer (Thinner 8). The patch was surrounded with aluminium tape, so that the thinner wouldn’t damage the 1920s covering fabric. After this a thoroughly Thinner 8 soaked fabric, which was covered with a sheet of plastic foil, was placed on the patch. The foil was taped tightly on the edges to keep the fabric wet and to prevent the thinner from evaporating. After a few hours “the package” was opened and the cotton or linen patch, glued with nitro cellulose lacquer, could be neatly detached. After this the edges of the holes without a patch were cut clean. Patches, with tattered edges sewn together, were cut completely open. Holes with tattered edges will not be sewn anew.

Now the patching of the cleaned holes could be started with Oratex-material. For each hole a larger round or rectangular piece was now cut out of the Oratex-material. After that the patch was glued and tightened on the hole by heating the patch with an iron, which was pre-heated to a suitable temperature, using a protective paper. The heating melts the glue from the underside of the patching material, fastening the patch to its place and tightening the fabric smooth.

This modern material is really handy to use and the patches turned out to be very smooth and tidy. There’s also the extra bonus of not having to protect oneself against the toxic fumes of the nitrocellulose lacquer. The appearance of Ortex- patches differs, however, from the traditional fabric patches with fringed edges from the 1920s. The work will continue in the spring season of 2023.

Photos by Lassi Karivalo.

Translation by Matti Liuskallio.