How can i inexpensively remove rust off bike before i donate it?

Back in like 2005 or 2006 My son as a gift got a brand new Schwinn Manta Ray

http://www.genesbmx.com/schwinn/mantaray1.jpg

The same one in the photo .Well in that period he used it maybe 3 or possibly 4 times.He was a tall child so on top of the look of a cross between a low rider & a muscle bike it boasts a super-cool frame that's brimming with attitude. The double-wide polo frame and extra-long Schwinn chrome springer fork give the bike a laid-back feel, especially when combined with the casual banana saddle and chrome sissy bar (otherwise known as a passenger backrest). The double-wide coaster brakes, meanwhile, help riders stop safely in any situation. Other details include a steel chain guard for safety, wide front and rear fenders extra Fat wheels.But I had the neck of the seat the( pole) customized for extra length.Here is the problem The Rust I have WD-40 To get it all aligned into tip top shape.But how to i get rid of the rust?its not rotted out

rust no whole or decay.Its from the elements of the weather.Cosmetic rust.The chain is great wheels great .The grips on the handle bars wil be the only thing that some person has to replace.I still have the original owner manual and registration card i never filled out.Once

i can get the rust off .I am going to post it on freecycle.org there are so many in need so any suggestion i sure would be grateful.Thank You Gsmom.