Hi all.
Apologies I know this will have been covered many many times but have had a quick search and just comes up with rotten chassis and the need to be replaced.
I’ve got a 2004 td5 with the usual rust on the chassis, it’s not totally rotten like many I’ve seen but does have lots of surface rust plus needed a small patch welded.
My question is could further rot be prevented by some product or to far gone to bother with so wait until it’s past welding and needs to be replaced?

Its a matter of judgement really, without actually seeing tghe chassis.
If the last patch repair was done well and the rest of the chassis is good, after removing surface rust, it could be worth introducing waxoyl and reapplying every couple of years, both inside and out, for a 3 or more applications.
Main area is the rear and I would remove the tank and prep that whole area, apply primer and paint externally before the waxoyl.
You could have a good chassis for many years but if its not worth saving you'll know as you go through the motions.

As @Bantam1 says prime, paint then Waxoyl or Dinitrol. I'm sure I've read people get good results with Linseed Oil but you'd need to research that for yourself

Waxoil is pish forget that...scrape the chassis apply fertan or rust bullet & apply bilt hamber wax.

Fertan – Rust Convertor

Rust Removal | Prevention | Anti-Corrosion Treatments | Rust Converters - Bilt Hamber

Rust paint, Rust Bullet, rust treatment paint rust inhibitor (rustbulletuk.com)

All available on ebay amazon ect.

You might not have to replace the whole chassis.
My bloke quoted me £700 to replace the rear half, or was it a third, or was it two thirds.
cannt memeber now!

Sounds like my chassis, good but age related rust and 1 small patch, iv come to the conclusion to avoid any paint as you can’t get every bit of it, eg top side and both paint and that nasty waxoyl only need a tiny crack or hole to allow moisture in and rust eats away behind it, you think chassis is good give it a tap then a great lump drops off.
What Iv been doing is scrape and brush it then apply a rust neutraliser, iv just used phosphoric acid as that’s the chemical most neutralisers use then spray wax inside, I use Morris Lubricants Anchor Wax then a tin of water and salt resistant grease again Morris make one called k99, designed for marine use warm it in bucket of water then paint it all over chassis.
Being grease it won’t chip off or allow rot underneath. Over time it dries a bit so not as messy when working on car as you may think.

Dynax s50 for the chassis inners.

Your vehicle's chassis sounds like mine when I bought it (except mine had no welding). From recommendations on here, I used Bilt Hamber Dynax-UB: https://www.bilthamber.com/dynax-ub
I cleaned off (using sandpaper) all the chassis that I could access and then applied this. I bought two 750ml tins (for brush application) and one spray cannister, using the spray for hard to reach places. It took me about four days. There was about 1/4 a tin left. I'm very pleased with the results.
There are more thorough ways of doing it, but it is a trade-off with the amount of time you have and your ability to get at the chassis. What I did was based on advice from Bilt Hamber, who, by the way, are really knowledgeable and exceptionally helpful.