commercial glues for aircraft building
Written by Victor A. Boyce, CSN

Recently I received a phone call from a gentleman wanting some information on Epoxy glues; the FAA had given him my number. This person was building Stearman wing spars and was looking for an Epoxy wood glue with a Specification number that approved the glue for use in Certified aircraft.

I told him that I did not know of an epoxy with a Mil Spec No. at this time and suggested to him that he use Resorcinol G-1131 which has several Federal, Military and Industrial Specification numbers, as used in the building of the Bellanca Viking, the same glue that we use to build Corby Starlet wing spars.

His response was that he had used Resorcinol before and he did not like it because it was weak and broke at the joints. Upon further questioning he told me that the glue that was given to him to build a spar was used in the building industry.

Indspec Chemical Corp. on being informed that I was building an aircraft, refused to sell me any glue other than Penacolite Adhesive G-1131 because this is the only glue they make that has been tested for use in aircraft construction. So there is the answer to the question "should I use off the shelf commercial glue".

Beware some commercial glues use an "extender" an excess of filler to make the glue go further for less money, in some cases this may make the glue brittle or weaken it.

Some glues require an amount of filler to improve their qualities, such as to reduce shrinkage as the glue dries thus preventing distortion of the job, however when more filler than required is used, it becomes an extender.

A very good resorcinol glue that is made in England is Aerodux 500, it is made in three grades, fast, medium and slow, select the grade to suit your workshop temperature.

Years ago when I was starting out in aviation I was building a glider and using Casein, a glue mixed with water, at the same time I was an apprentice Carpenter and Joiner working in a wood shop.

When Casein glue was mixed in the shop, water was used as an "extender" in other words excess water was used to create more glue which was used to make doors etc.

I was surprised, obviously I could not do this on the glider that I was building because it made the glue weak. Casein glue ­ now obsolete for use in aircraft ­ was at that time in use.

Later on in my aviation career I found out just what happens to Casein glue to make it fail when used in aircraft construction and repair. When repeatedly wet by rainwater as when the aircraft is left outside, the microbes in the water devour the glue in the joints, leaving only a brown stain where the glue was. Even though the best Casein glue contained a fungicide to prevent this from happening, and the wood varnished, the microbes eventually overwhelmed the fungicide and the glue was lost.

I spent many hours of my early career re-gluing joints in wooden aircraft especially along wing trailing edges and bottoms of fuselages where water would lay when drain holes became blocked by dirt etc.

So do not use just any glue on your aircraft, do not be fooled by a cheap price, get the best that you can get.