Assuming that you are looking at the furniture boards in the diy stores, are they definately pine? Could well be spruce which is stronger than pine. Either way, give it 5 or 6 coats of a low VOC varnish and it should seal up nicely. Make sure that you run the heating in the viv for a while without anything living in it. A lot of varnishs will release more fumes when heated even when they appear to have finished doing so at room temp.