This is a complex subject and there will be many schools of thought. Here is mine
I agree with TnTkr in some respects its already too late to simply 'underseal' a 6 year old car. Especially on a Honda, who's reputation is more for mechanical reliability than rust prevention. It would need to be carefully checked over and any rust found expertly repaired , and then the cost of the rust prevention treatment itself. Done properly, by an expert l , this could cost many hundreds of pounds. This money might be better spent on replacing the car sooner, before the rust becomes an issue. But I accept that frequently replacing a car can be a stressful experience.
I'm not saying ignore rust. It should be expertly repaired as soon as any is found. Vulnerable areas, that are hidden from sight, might benefit from an extra coat of paint , and there are various waxy oily type rust prevention treatments. (My experience dates back to the days of 'waxoil' but there are better types available) . Such treatments can certainly keep rust at bay, but is more economically viable if you are able to do it yourself. And many of the waxy type treatments can get washed off and need regular re treatment. Some treatments involve drilling holes in box sections and injecting treatment into hidden areas. . Expensive, and maybe already too late.
If by 'undersealing' you mean the traditional treatment of spraying the underside of the car with a thick coating of bitumen type 'underseal, beware. If an MOT tester suspects the coating is too thick and might be be disguising areas of rust ,or inadequate 'bodged' repairs, it can fail the MOT. A 6 + year old jazz with shiny new coats of underseal will be suspect. Also when you eventually sell the car 'savvy' buyers may also be wary of what a coating of underseal might be hiding.
