In order to properly conserve the painted fairings and cowlings, extensive testing was conducted for each phase of treatment. A proposal was then written, and all treatments were discussed with the curator to ensure the proper preservation of historical evidence and to produce an accurate representation of the Horten. Additionally, all components were fully documented with written and photographic documentation before, during, and after treatment.
A few principles guided the selection of our materials and methodologies included:
Once analysis and documentation was performed, extremely careful cleaning was conducted. Loose dust, dirt, silica gel, plant material, and corrosion on the surface was removed with a soft brush and a Nilfisk Vacuum with a HEPA filter. Water soluble dirt and residual dust was removed by wiping with a soft cloth and deionized water with a surfactant (similar to a soap) called Triton XL 80; this was then rinsed with deionized water and patted dry. Oily dirt, grime, and old preservative coatings were removed with mineral spirits with soft rags, or cotton swabs in sensitive areas.
Corrosion was mechanically removed with scalpels, abrasive pads, and wire brushes where it was thick. The surface was wiped down repeatedly with mineral spirits to remove loosened corrosion, followed by dilute phosphoric acid to further loosen corrosion along with mechanical action. The surface was rinsed with deionized water and dried with paper towels or cotton rags to reveal stable areas of metal or stable corrosion products. Care was taken to not remove any evidence of mill scale. Once all loose iron oxide was removed, the surface was cleaned with vacuum and brush to ensure a lint and dust free surface.
Areas of exposed metal were treated with 8% tannic acid in deionized water with 2% ethanol and phosphoric acid (following the Canadian Conservation Institute recipe). The tannic acid was applied to the surface with a stiff bristle brush. Once the tannic acid had reacted with the metal surface and was completely dry, the surface was rubbed with very fine steel wool to expose any areas where the tannic acid had not converted the metal. The surface was then vacuumed and the tannic acid was reapplied in locations where there was remaining powdery corrosion. Excess tannic acid, especially in regards to the painted surfaces, was removed with cotton or paper towels and deionized water and thoroughly dried.
Staining from corrosion residue and brown preservative coatings was reduced with a solution of Versene 100XL (Na2EDTA in NaOH) in deionized water (1:3) by gentle swabbing on the surface followed with thorough rinsing with deionized water. This was extensively tested underneath the 3D microscope to determine best methods to remove the staining material without harming the paint surface.
Exposed metal surfaces were coated with 30% Paraloid B48N in m-Xylene with fumed silica added as a matting agent. Paint surfaces were not coated, however one to two coats of 15% B48N in m-Xylene was used to coat painted areas with extensive small pits where it would be impossible to apply the coating on only areas of exposed metal. Flaking paint was also tacked down with the 30% Paraloid B48N in m-Xylene solution.
The consolidant and coating selected for these areas was Paraloid B48N a methyl methacrylate and butyl acrylate copolymer with an adhesion promoter. This material was chosen due to its stable aging properties, areas coated with it can be re-treated with paints or other adhesives, is more easily removable than waxes or other polymers, and its wide use in metals conservation already. This material's solubility parameters were also compatible with the solubility parameters of the existing Horten paint, meaning that it can be removed without harming the original paint.
Some metal components had rusted so extensively that material either no longer remained, or in some areas resulted in very thin and fragile metal. Areas of thinned metal with losses were supported with a thin layer of Hollytex roughened with an abrasive pad to promote adhesion and applied with 30% Paraloid B48N in m-Xylene. In areas of large or complete loss, a layer of Hollytex was applied first to conform to the edges followed by one to three layers of Varaform sheet was applied, adhered, and set in place with heat.
Hollytex, a white, non-woven, synthetic fabric is 100% polyester with no known extraneous binders or resins. It was chosen for its long term stability, ability to conform to surfaces, and strength.
Golden acrylics were used to visually integrate the fills, however were toned so that up close the replacement material could be identified.
Photographic and written documentation was then completed post-treatment in order to establish a baseline upon which to observe changes in the Horten over time.