What is HDO Plywood?

What is high density overlay plywood?  Beekeepers have been using pine to build beekeeping equipment for well over 100 years and it is by far the most popular material for hive components.  That said, it's not the best material for everything in my opinion, only the most popular as it is much more readily available than high density overlay (HDO) plywood.

High density overlay plywood was originally developed for sign makers who would make exterior signs out of plywood and needed it to be very durable over time. It was later adopted by concrete contractors to use in their concrete forms.  If you have ever seen those metal concrete forms in use, they have HDO plywood linings in them which will release 100's of times off the concrete they pour versus a handful of times with regular plywood.  Some commercial beekeepers purchase used HDO from concrete contractors (some concrete contractors refer to HDO as "steel ply") after they replace the HDO surface on their forms and then repurpose those used pieces of HDO into bottom boards and lids.  That HDO is used, though, and has holes in it, gouges out of it, concrete remnants on it, etc.  We purchase new HDO plywood in bulk and use it to build bottom boards, lids, double screen boards, etc.

HDO plywood is made with much more resin than standard plywood and is bonded under heat and pressure, which makes it consistent in dimensions, stronger and heavier, and limits warping. The appearance of it is closer to a cabinet grade plywood than CDX.  The visible outer layers are the high density overlay that give this plywood its name and are highly durable resin-impregnated fiber which help to protect the plywood from moisture which is a very desirable feature for a beekeeping equipment material. 

Bottom boards and lids take the most abuse in a hive as far as moisture and rotting is concerned.  We recommend and use HDO bottom boards and lids with pine boxes and find this is a good compromise between durability and affordability of the hive.  Preparing HDO equipment for use is pretty similar to pine equipment, and we recommend using an oil-based primer and a quality acrylic enamel paint topcoat.  It is very important to properly seal any exposed edges of the plywood with primer and paint, but once the edges are sealed you have a sealed envelope around the wood plywood that repels water with the HDO outer layers and the sealed edges.  We expect HDO equipment that is properly primed and painted to last 15+ years versus 5-7 years compared to pine.