As The Wood Chisel Carves:
Hi! as the wood chisel carves is a new feature I hope to update daily or weekly about sign making or wood working in general.
This weeks issue is about what is a better backing for a large dimensional sign PVC backing or an exterior plywood backing. Your probably asking yourself what is a backing and why do you need it? The answer is while not complicated is not a simple answer. A backing is a material that I glue to the back of a sign to both stabilize and to provide a fastening point for mounting. I like using a backing on wood signs to keep the wood from moving around and to indeed give the sign a sound foundation to build upon. If a thick enough lumber is used, a backing might not be needed at all. However if no backing is used the wood planks must be edge joined in a very strong manner such as with shaped edges splines or mortises. If the edges are joined in this way the carved wood sign will shrink and expand as a unit and the joints will stay tight. This method however does have its drawbacks, Most of these drawbacks involve economy and not function, however these would include the fact that if a backing is not used the first uneconomical price is that the wood must be thicker to accommodate the edge glue, shaping splines or mortising this results in a more expensive sign to the customer. Also particular care then must also be taken in the selection and alignment of grains to prevent wracking and warpage. When constructing HDU signs it is almost always advisable to include a backing of some type to provide strength to the finished sign as well as to provide a place to fasten the sign to its mounting post. This is where we come to the dilemma of whether to use a plywood or PVC backing for such a sign. For wood signs my opinion has always been that a plywood backing is best for this wood to wood joint, and that a waterproof outdoor glue be used. For HDU signs the equation is somewhat different, Why would you want to fasten an almost indestructible sign to such a destructible material such as plywood? It just doesn't make sense does it? With that said the best material I have found to take on the role of backing for an exterior dimensional HDU sign would be a backing of 3/8 to half inch flat PVC joined with a suitable building cement or compound. I like 3m 5200 for the more expensive exterior HDU or carved wood signs. But a less expensive construction adhesive such as liquid nails construction adhesive or an equivalent can be used for the less expensive dimensional sign. PVC can take a wood screw or a threaded fastener to fasten it to the sign mount or poles. This type of PVC backing is impervious to rot, insects and the effects of most UV and weathering. So there you have my opinion on the subject of sign backings and what they should be made of. My first choice of backing for carved wood sign would be marine cored plywood my second for this type of sign would be regular exterior plywood. For HDU signs the first choice would have to be PVC followed by marine core plywood and then by exterior plywood.