Quite by accident, I ended up covering a card part with white glue (PVA). When it had dried, it had dried clear and the colours of the part were not affected but I had a clear protective skin over the part. I then tried it again on another printed part but this time slightly diluted with water. It painted on perfectly (it looks unsightly until dried) but it dried to a smooth, clear, "gloss varnish" effect. This is great for reinforcing the glue lines and protecting the coloured ink on the parts. I don't know the long term effects of the glue - yellowing with age, etc, but I am going to use it to protect my models.