Sorry for resurrecting an old thread, but maybe others will find this useful one of my other passions in life is railway preservation and have helped preserve and restore several wooden bodied railway vehicles. and as the cost and availability of sheet timber in the sizes required is rather prohibitive, we tend to use plywood to replace life expired paneling. The problem with plywood is that if moisture gets in between the laminations, over time it will start to delaminate painting the edges with paint or varnish will help to reduce moisture ingress but as this only covers the surface any damage to the paint will allow moisture to get in. So to overcome this problem we use the following method. First mix a 50/50 solution of spirit based varnish and white spirit and soak the ends and sides of the plywood sheet keep doing this until the plywood will absorb no more, the white spirit will wick in between the laminations pulling the varnish in with it then leave for a couple of days to dry. then using a good quality yachting varnish repeat the treatment using a 25% varnish, 75% white spirit mix, once this has dried give it a couple of coats of 100% varnish, also it's a good idea to treat any knots on the sheet the same way. If you are going to paint over the varnish do remember to sand the varnish first to key it, you would be amazed how many people I have seen forget this step and wonder why there beautifully painted waggon starts to peel. one word of warning some very cheap plywoods use cheap glues to bond the laminations and the white spirit can break that bond causing the plywood to fall apart so try on an off cut first. and please don't ask me how I know that. this method was shown to me by a carriage restorer called Steve Middleton and a gallery of his work can be seen here https://www.statelytrains.com/index.html Cheers Colin