Beaumiller -

I would go ahead and do the fabric and dope or water-based poly on the models you mentioned - especially the Liberty Sport and King Kobra. I think dope is a little lighter overall than polyurethane. Your real weight comes in with colored dope.

Use clear nitrate, with light sanding of 400 grit between coats, with maybe a little talc filler. You may look to trim some wieght from the airframe to compensate - older kits seem to usually be overbuilt anyhow. A good sanding job and fabric base preparation makes all the difference, and you'll use much less final color. Things start going downhill fast when you keep adding the colored dope or paint to even out a poorly prepared finish - that could be a killer on something like your Liberty Sport.

I did a Top Flight Contender .40 sport/pattern plane with Koverall and dope and it came out great weight-wise. Try the fabric and painted method - you'll never go back to shrink wrap again! (kidding - I use Monokote too - depends on the project.) By and large, for a large or a complex building project like a biplane, or any scale plane, fabric is the way to go for me.

One other thing I forgot to mention. Thin CA is great for getting the fabric to stick down in those hard-to-reach spots. Be careful with it as it can make a mess, but for wingtip edges, or wing saddles or hatch edges, a little thin CA rubbed in (keep moving so your finger doesn't get stuck!) bonds the fabric down forever. I also use it around wing-bolt holes or strut-attachment points for a durable job.