At what point is a plane too heavy to fly? And why is it too heavy? If I'm building a kit that says 10 to 12lbs what happens if it's over that?

It depends on the plane. A heavy trainer is better than a heavy cap or g-202 or such. They all do different things. The weight on the box isn't always that accurate. Expert builders might hit that weight, but not always.

What plane is it and how far over are you?

If it says 10 to 12 lbs aim for 10 lbs. If possible, aim for lower, 'light is right'.

It is a TF Cessna. Its not finished yet but I'm figuring 14 to 15lbs when completed. I was planning on painting it with 2 stage automotive paint. I'm hoping the paint wont be more then 3lb's

Three problems exist for the heavy plane. First, the heavy plane lands fast. This is due to high wing loading, which effects the glide ratio tremendously. Second, because your glide ratio is reduced, landing under dead-stick conditions takes enormous skill, sort of like landing the space shuttle. Last, your choice in power plants is greatly reduced, subsequently reducing the overall flight performance that could be had out of the plane. Heavy military fighters are fine and I enjoy them very much. I like the challenge of flying them off the pavement and back down. For pattern or acrobatics, heavy plane are dissapointing to say the least.

3 lbs. of paint is a LOT of paint for a model of that size, even if you use a two stage automotive type paint. You should not add more than just a few ounces when painting a model of this size. Any more and you are applying the paint way too thick. Remember, you want only just enough paint to provide even coverage. Any more than that is a waste of paint, money and weight.

To your original question though. If you end up over weight on your model, you can expect it to have lower performance in climbs and handling than if light. It will not respond as crisply, or smoothly as it would if built light. As stated by others, it will both take off and land faster. Faster landings are harder on the air frame and landing gear in particular. You will also have to fly with more power which means the engine has to work harder and uses more fuel. Prop choice becomes more critical as you need to choose one that will fly the model fast enough to be comfortable in the air, but this usually means a sacrifice in pulling power for take off's and over shoots.

John

Easy on the paint and finishing. That seems like a lot of paint to me too. Are you glassing it too? Hopefully you won't add that much finishing it.

A little bit of a side note, but if you fly with a lot of wind a higher wing loading (heavier plane) will handle the wind better (i.e. it won't get bounced around as much) than a lighter plane....maybe this matters, maybe it doesn't depending on your typical flying weather...

Your stall speed will also be higher, which was eluded to, but not mentioned. The higher wing loading of a heavier airplane will make it more prone to tip stalling and spiralling in, the reason that landing speeds MUST be higher.
Dennis-