Cleaning/painting the underside

The underside and wheel arches need making good. I bought a couple of tins of Hammerite Underseal with added Waxoyl, plus six spray cans of the same. So do I sand down as much rust as I can before using this stuff? I don't want to make things worse (although that's entirely possible!).

Cheers
Pete

Hi Pete,
I would knock off any loose rust and paint, treat with a rust converter and then apply your selected topcoat.
Typical converter - plenty to choose from;
www.frost.co.uk/granville-heavy-duty-rus...bd98760c8c9428ec6daf

Doing anything is better than doing nothing.

I strongly advise against the use of any type of "rust converter". Most of them are basically tannic acid.

To make the elaboration simple, two main reasons against tannic acid:
1. It rarely penetrates the rust more than its shallow surface, thus leaving its inner layers untouched.
2. The converted top layer of rust prevents the corrosion inhibitors, which are usually a part of the subsequent coating like a wax or a paint primer, from performing their job, because they do not have contact with the untreated rust below the converted top layer.

The best rust removal method is mechanical sand blasting. If not possible, then a wire brush, followed with abrasion with sand paper (to roughen the metal which the wire brushed has polished) and then phosphoric acid to finish off the remaining small rust flecks.

Here is some compiled knowledge:
www.bigjimny.com/mediawiki/index.php?title=Rust_protection