Sure.

First make sure the metal is structurally sound and take a wire brush to it. Remove as much rust and undercoating as possible.

The next step you have a choice. My father would just slap POR-15 over the wire brushed metal and call it done. And he'd probably be right, that stuff is magic and will last a good long time.

However, if you really want to be thorough, I'd make sure ALL rust was gone first.

To do that, go to a good Mom 'n' Pop hardware store that carries everything ever and buy some Phosphoric Acid. BE CAREFUL WITH IT! Yes, you drink it all the time in soft drinks but it's a slightly different concentration. Carefully brush it on the rusty spots and let it drip dry. Do this a few times. Even serious professionals will use this stuff to remove rust, though perhaps in a proprietary formula with some additives.

After the acid has eaten away the rest of the rust, wash it down with plenty of water, maybe with some baking soda mixed in to bring the pH level of the metal back to neutral. Lots of water is probably fine.

Then paint the metal with something, either a rust proofing like POR-15 or a primer and some Rustoleum.

After that, slop on some undercoating. You can buy that in any auto parts store pretty cheap, you could probably find some way to make your own if you're that hard up for cash but I wouldn't if I were you. Left over driveway sealant maybe?

It's not hard. But be careful. Wear rubber kitchen gloves for any chemicals. Wear goggles and a dust mask while brushing. And if you're adventurous and actually decide to chemically eat the rust away with acid, keep a garden hose or big bucket of water nearby incase you get some on you. It can cause serious burns if left on.

Be careful, good luck.