David. Matt finishes mark very easily and so a coat of varnish assists 9in keeping things tidy and tougher in the long term.

I woild not have thought the yellow primer was porus as such, not for model boat deck use. Again, it is not very tough so clear matt varnish keeps it safer.

I use red brushing iron oxide paint below the waterline and have not had a second thought about having to varnish it.

Ashley

Polyurethane or that sort of varnish will darken and yellow over time. I have been using water based varnishes lately, nothing special…Ronseal or Wickes/whatever. These do not yellow and I can vouch for this as the door to my living room was acrylic`d on one side and polyurethaned on the other and you can see the difference…after 25 years.

The waterproofness of "interior" varnishes should be taken with a pinch of salt for model boat use. Interior use means (obviously) not exposed to all weathers outside. It is perfectly waterproof enough for a bit of a splash now and then on a model.

Ashley