17 December 2006, 09:39 AM
One and all,
It has been oft repeated here, and in other related threads, that the doping regime for the Dr.I was 1 coat of clear and 1 coat of color followed by 1 topcoat of varnish. Since the doping protocol in use bears on the question of the camouflaged appearance of the finished triplane, the "1-1-1" regime -- which tends to serve as an argument in favor of the current convention of "olive brown over CDL" as opposed to the multicolor hypothesis -- merits more scrutiny than it has been given, as far as I have seen.
In the present context, the common meaning of a "coat" is the application, in a single pass, of a single layer of a coating such as paint or lacquer or stain, which must dry or cure before the next layer can (or should) be applied. In this sense, a single coat of dope -- whether of period nitrate or contemporary butyrate, whether on traditional organic fabrics like linen or modern synthetics like Ceconite -- is simply insuffcient to achieve the functions for which it is applied, namely, shrinking (tautening), sealing, protecting, and strengthening the covering.
With respect to modern practice, a current guide for the application of dopes from Randolph Aircraft Products, one of the premier Amereican manufacturers of aviation coatings, advises the following regime of 14 coats:
3 coats of clear nitrate (nitrocellulose)
3 coats of (transparent tinted) butyrate
4 coats of (anti-UV pigmented) butyrate
3 coats of pigmented butyrate (color coats)
And it should be borne in mind that this regime is for application to Ceconite, where the function of tautening the fabric is not required (indeed woulb be detrimental).
As for Fokker practice, consideration of the relevant period documents, especially of the wing materials list, is consistent with the application of multiple coats of the (Emaillit) dope -- at least 4 and probably more.
As a lacquer coating system, the application of each new layer (coat) of dope melts and binds to the previous one, in the end creating what might be called a monolithic composite "film". However, to use "coat" as a synonym for this final film is confusing when trying to reconstrut the properties of the streaked camouflaging of the Dr.I and other Fokker-built aircraft thus finished. The same caveat might also be appropriate when considering the finishes of the Eindeckers, including the E.V, where the supposition of single coats of dope or dye have also been advanced.