I agree with both of the above statements. Personally, I use acrylic clear top caoat designed for cars, I find it goes on well and does a good job of preserving the model. I had one which 'went awol' and I found later a young nephew had taken it outside for a 'play'. It lasted two days with some light rain and no damage. (Admitted it was under a tree as well). Good Luck!
Yellow color changes in paper model can be caused by light reaction and residual acids from manufacturing.
Choose an acrylic fixative that is non-yellowing. Blair and Krylon have products to do so but first try them with a test printout to see if application might cause colors to run. It can be a matter of light spraying for the first couple of passes before successive saturating reapplications.
I have used polyurethane clear coats but found it yellows under the fluorescent lights in my office. I would recommend sticking with non-yellowing acrylics. Save the polyurethanes for treating wood in display cases and stands or for models where yellow tints are not problematic
Paper pulping often involves sulfuric acid treatments. If the acid residues remain un-neutralized, the paper can deteriorate over time. There are acid neutralizers available. Krylon markets "Make It Acid Free!"
There is also a chemical on the market, called G4 and G7, basicly they are the same, with the exception that the G7 also protects against UV. Basicly you paint the model (or the subasemblys) with it, making the paper extremely hard and waterproof. I even know a guy who builds the hulls of his RC ships paper-over-a-wooden-frame.
Sig Nitrate Clear Dope. Paint parts before you assemble and your touch-ups/edge paint w/not bleed. This is a paper preservative used to fix the paper/silk span to balsa models. Works great and takes paint well.:thumb:RH
Hi team. If I could expand on the question a little. I have a badger spray gun (used to use it on my plastic models) which now sits idle. Is there any that you would recommend for use in a sprayer as opposed to off the shelf spray cans.
I work in a museum and deal with this type of conservation issue daily. The yellowing is caused by the inherent acidity if the paper. Top coats are a bandaid approach that do not address the real problem, and the deterioration will continue. There is a deacidification liquid on the market called Bookkeeper, it is available through many archival supply houses. One of the better companies is gaylord.com (legitimate site I swear) Unfortunately it is not cheap. It neutralizes the acid with magnesium oxide and leaves what is called an "alkaline buffer" to act as a reserve to neutralize any new acid formation. It needs to soak into the paper, so if you have done any varnishing it needs to be used on the back side of the paper.
The brittleness can either be from the acid deterioriation, or could be from UV light. UV light wavves are very tight and can actually sever molecular bonds causing the object to literally fall to pieces. If it is UV damage you can halt the process by storing items in total darkness for at least a month (6 months is the museum standard) to halt the molecular excitement caused by the light.
Unfortunately, either way, the damage is irreversible. All you can do is arrest the development of further damage from these sources. As is posted above, the best approach is prevention, acid free paper, glue, paints and UV protectant top coat.
Hope this helps out, sorry it is such a long post.
Bagpiper, the Bookkeeper deacidification spray also comes in a pump version as well as spray can. You could spray it with your airbrush for more controlled coverage, but it just adds a step from using the pump sprayer.