What are the reasons large warbirds get fiberglassed instead of primed and painted?is it to make the plane harder and stronger?im not sure,do people just prime and paint planes instead of glassing?

glassing doesn't take the place of priming and painting... You still have to prime and paint it after the glassing... Generally it's because of the smooth shapes and some of the custom work that has to be done to get the cowls to fit perfectly around the engine, etc... Glass is also much stronger than any sort of plastic or anything else...

I understand you still have to prime and paint,was wondering if you could just prime and paint without glassing,or if its mandatory to glass a large rc plane,or use some type of covering.

Just priming and painting the wood will work most often, but it is hard to fill the grain and you get a lot of stress cracks in the paint over time. Glassing takes care of a lot of that and makes the airframe stronger in most cases.

It depends.

If the airplane has any open structure, it would of course have to be covered with SOMETHING.

In the case of open structure, that would usually be a shrinkable film or fabric. Some need paint and some don't.

If the airframe is completely sheeted, you could just seal, prime, and paint, but giving it a layer of fiberglass will give it that nice smooth, well blended look. It will also remove any sign of woof grain and make the plane look more plastic or metallic.

And it's not whether the plane is large or small, it's more a matter of personal preference.

glassing always makes it stronger,

like mike said (lol like mike [just made me giggle while typing]) if it is
full sheeted, glass it!
Fiberglass also looks shinny, you want want to paint it!

Thanks everybody!

I had a chance to work on an old Top Flite Corsair. The plane was primed and painted, no fiberglass. The plane was probably 10 years old and really looked it's age. Just about every square inch of the thing had hairline cracks along the grain and some were even pretty big. If he would have put a light layer of glass on the plane before paitning the cracks and splits would not have occured. Sure it would have weighed a bit more, but the finish would have lasted. There are quite a few planes that I have seen that are clear monokoted and then painted over.

Primary reason for glassing a kit is to get a smooth, stable surface that is ready for priming and painting. This results in a good, long lasting finish. Strength is a secondary side benefit. Usually, one tries for a very light coating to avoid weight. Only one layer of very light weight (.5 - .75 oz/sq yard) fiberglass is used, also, as little resin as possible to do the job.

Scott