Borate insecticides are excellent non-toxic products for application to untreated and unsealed wood members in new or existing covered bridge structures. Some commercially available formulations use glycol as a vehicle to distribute the borates beyond wood surfaces and into structural joints. Products may be spray or brush applied (per manufacturers recommendations) once wood surfaces are clean and dry. Coatings cannot be effectively applied to wood that has been treated or sealed.
Borates have also been known to perform as fire retardants to some extent and may be beneficial in this regard as well.
The only drawback to the use of borates is their solubility in water, i.e. they can be washed
out of wood surfaces if exposed to constant water applications such as rainfall, etc. Therefore
these products are recommended for interior surfaces or under-deck applications.
Various commercial products are available as fire retardants for interior and exterior applications. Exterior use products should seal wood surfaces to prevent wetting but should also allow surfaces to age or gray to natural appearance. Retardants used over borate insecticides should be compatible as overcoats.
All coatings, whether fire retardant or insecticide should be applied as clear solutions. Clear solutions which contain salts sometimes dry with a very slight milky (surface) appearance, however the benefits of these product far out-weigh any such considerations.
[This article submitted by John Weaver, P.E., Vermont Agency of Transportation
Originally posted December 2, 2001]