would it be a good idea to sheet the whole plane with balsa wood, or monocote it instead? the reason why im asking i wanted to finish it with fiberglass cloth then prime and paint it. what do you think???

If you're going to fiberglass and paint it, it will have to be fully sheeted. But be careful that you're not adding too much weight.

I can almost gaurentee that if you sheet/glass/paint a plane, it will be heavier than iron on covering. This is especially true if you must sheet areas that originally were not supposed to be sheeted. Many planes can tollerate a little extra weight, but you need to be careful you don't add too much weight. Also consider where you are adding weight. PLanes are not symetrical and have more surface area behind the CG than in front. This means there is a good chance your CG will shift aft after you sheet/glass/paint. This is no big deal, but you just need to be prepared for that.

Just curious, why do you want to paint the model? What plane is it?

There are ways to minimize the amount of weight added if you sheet and then fiberglass.

Do a search on fiberglass finishing. this has been discussed many times.

If you do fiberglass correctly it is no heavier than monokote. You have to keep the finish thin by sanding every coat to leave only what is absolutely necessary.

Also, as ho2zoo said, you can only glass sheeted surfaces. If you want to use paint, but don't want to sheet, then you can use tissue, silk, silkspan or coverall with dope. Doped finishes are also light and don't take as much effort to keep them light, but they're still a lot of work. In my opinion a painted finish is much superior to film, but they are so much work that I only paint "special" aircraft - especially considering I'm such a lousy pilot that my planes don't live too long.