The problem isn't in the covering, its probably the wood selection, likely a luck of the draw thing, like most kits. Koverall and 5 coats of dope is at least as light as any film covering. If you start glopping house paint or epoxy , or automotive paint all over it, that changes things though. Anything less than 7 lbs on your LT 40 will fly just fine, that thing has a huge wing area, never saw one that didnt' fly well no matter what it was covered with, and on any ol plain bearing sport .40 as well.