If the fins on your radiator are bent, you can try to straighten them with a fin comb. This is a tool that looks like a small brush or rake, and it's designed specifically for this purpose. You can find them at most hardware stores.
To use a fin comb, start by working on one section of the fins at a time. Gently insert the comb between the fins and then pull it through. Be careful not to bend the fins any further as you do this. You may need to repeat the process several times to get all of the bends out.
Once you've finished straightening the fins, make sure to clean any dirt or debris out of the radiator before putting it back in service.