Hi Sportflyer

I've used Shellac for several years in this type of application. It is not so much a problem of weight but if your thin layer of epoxy disappears into the soft balsa you have a dry joint. I've seen a total de-lamination with drastic results when this has not been done. Tarmac is not soft. If you are worried about strength some sealing is possible by mixing a small quantity of Microfibers into the resin, West System Number 403, but you are still not sure how good the joint is?? I thin the full strength shellac about 30% with Methanol. As it is alcohol based it dries in about 30 minutes in a warm room, I spread it with a bit of cloth, quicker than a brush but you end up with furniture colored fingers for a day or so.

Andy