A rather simple technique with a very easy to use material. Future Floorwax, found generally at a store such as Wallmart. Most if not all clearcoats will eventually yellow, especially those that are solvent based. Future is a water based product.
How do we apply it ? Easy. I have purchased a box full of small Dixie Paper Drinking cups, I think 2 oz's. I por Future into the small cup, and with a very clean white cotton cloth , apply the Future to the model by wiping it on in sections with long even strokes. Do not let it drip. You do not have to do the whole model all at once. do it in sections, using any body edges or roof lines to create the separation point. Future takes about 10 minutes to dry to the touch. By the way, while you are applying it, it may appear uneven, rest assure, it will smooth out on it's own. Liquid seeks it's own lowest level. Yes, you can put a few coats on but wait a few hours between coats. You can even sand with the polishing cloths between coats if you prefer.
Ok, now another advantage. The Future coated model is now protected from dust and enamel paints . I recall one time I had just completed a Nascar model and wiped it down with Future. By accident, I spilled an entire small bottle of Flat Black paint on the front hood area of the model. I wiped the paint spill off with thinner and low and behold, no paint damage whatsoever. Acrylic based ( water) dries very tuff and enamel thinners will not harm the final finish.