RUST you have seen nothing check out my new topic. Babs

I've scraped loads of good welsh mud from my inner rear wings, and given them a good brushing over, whats best now?

A final clean and some underseal? I'm just concerned about the surface rust, is it a bad idea to seal that in? I don't have an angle grinder and wire brush, but could get one if necessary...

I reckon, if it isn't rotten, leave it alone.

As Viking has said, they lasted this long without needing work, as soon as you take it back to bare metal it'll start to rust, we have to paint everything we shotblast at work within 2 hours, otherwise the warranty on the paint is void. This is because sitting in the open atmosphere it will get moisture into it and it'll start to rust.

my renault 4 was almost the same underneath as yours although not muddy just surface rust
i used dinitrol on mine
wire brushed it 1st then sprayed the rust converter on the surface rust,it turns it black and 1 can goes a long way
after a week or so i then used their underbody wax over the top
bilt hamber also do a similar line in products

much better than any type of underseal as once youve covered it up under there you dont know theres any problems until a bit of floor drops out
the wax is see thru as well as the rust converter so you can keep your eye on it and retreat when needed
got to be better than just leaving it unprotected,you dont want to end up with floors like my ami!!
mattt

Waxoyl - or similar products - are, in my opinion, the very best materials to use.

They contain rust-killers, but the most important characteristics they have is the way they (a) never 'set' or peel, (b) they COMPLETELY repel water, and (c) if thinned before application it will 'creep' into every tiny crevice by capillary action.

(How much to thin the Waxoyl depends on where it's to be applied. If you are spraying through small apertures, inside panels and hollow sections, or onto seams and joins, then thin it MORE than the recommended 10%. Then apply further coats the following days if you want. Onto flat areas such as the floorpan and body panels, then keep it pretty much neat. Waxoyl can be applied very successfully using a cheap 'airless' electric sprayer.)

First remove any loose rust and underseal - just use a scraper followed by a wire brush. Then apply any of the well-known 'rust-converter' products to convert the remaining rust layer into a stable 'phosphate-based' coating. Apply a metal paint over this if you wish to make it look neat, but don't use Hammerite as it's too 'brittle' and will chip on impact. I use satin-black quick-drying (water-based!) 'direct-to-metal' paint that I obtain from Wickes! However, this paint isn't actually required if you use 'black' Waxoyl - this has a nifty finish on its own.

Use an extension probe - preferably flexible - to get into every box-section, nook and cranny. And spray 'til the cows come home - and the stuff dribbles out the holes. Waxoyl is cheap - around £25 for 5 litres - so don't skimp.

When spraying wheel arches, sills and similar, bend the sprayer nozzle so's you are spraying 'back on yourself' and getting the stuff onto that hidden return edge flange. Hold a piece of card up against the outside to stop excessive spray going everywhere, including your face...

Either remove doors panels (preferable) or use your flexible extension nozzle, but get the whole of the inside thoroughly coated. Also stick the nozzle up inside the door window frame section and blast away with thinned stuff.

And, for pity's sakes, wear a mask and goggles.

Other points to note are: the coating on exposed areas such as wheel arches and floorpan, which will be subjected to driving water spray, will likely be slowly worn away over time, so apply further coats here every couple of years. And, either drive the car only in dry weather for a couple of days afterwards, or else don't drive it at all until the Waxoyl has at least partially 'set'.

As for the OP's particular case, don't bother trying to scrape off any firmly attached existing underseal, but do coat over it with Waxoyl - this will help to keep it flexible and prevent it from cracking over time.

I use black bitumous agricultral paint.

Scats do it at five litres for a tenner (enough to do the floor pans inside and out on a 2CV thick enough to hide all manner of sins). Or if you have want to hide something seriously bad you can get twenty five litres for forty quid.

Layer it on as thick as you possibly can, ideally in a couple of coats if time allows. Its fairly tacky even when dry and not ideal for the inteior, but it makes even cardboard look just like steel to the MOT guy.

Naturally though I would only advocate using cardboard and hot melt glue for temporary repairs when you have run out of plate to weld in.