Underbody rust repair

jrrowleyws

My car has a patch of surface rust on the subframe (about the size of 3 £2 coins). Its currently very superficial and I'd guess that it is a result of a garage jacking it up and damaging the underseal on the car.


I was wondering which products would be the best to repair this. I will sand off the majority of the rust and then apply a rust remover (I have one made by hammerite which works OK). Once I'm down to bare metal, I'm assuming I need to prime it? I have some Halfords red primer in a spray can, would that be alright?


Seeing as its underneath the car, once its primed up, I'd like to put some sort of coating on it to match the rest of the underbody (the rest is covered in a rubbery type substance). What should I use? Is Hammerite underbody seal any good? Would Hammerite Stone Chip Shield be better?


Essentially, I'm after a practical repair which will hopefully hold back the corrosion fairly well.


Any advice welcome

Crabman

I've heard good things about Bilt Hamber's hydrate 80 and deox products which are apparently excellent at removing rust. I think etchweld is the name of their primer.

I've never used them myself so can't say how good they are, maybe someone here can comment if they have had experience with that brand.

I'm not too convinced by Hammerite products, their anti-rust primer doesn't seem particularly effective. A family member's car has had rust come back through several times after using a Hammerite anti-rust 'kit' comprising gel, primer and a wire brush.

Joe_Horner

I spend a fair bit of my time holding back rust (one of the joys of classic car ownership without classic car budgets)

By far the best treatment I've found for underbody is Dinitrol cavity wax followed by a coating of their underbody wax.

They're not as "permanent" as a sealant or stone-chip, which means you really need to check and patch up as needed yearly or so. But they have a few very big advantages for anyone applying them without full bodyshop facilities:

They'll "stick" to metal even if it's not scrupulously clean and will even displace a certain amount of surface dirt themselves.

They "creep" into any seams or bits that you miss with direct application

They have fairly effective rust killers built in, so there's (genuinely) no need for pre-treatment beyond a good wire brushing.

They can't "separate" from the metal as rubberised coatings can, so you won't get water traps being created by the protection itself

They're transluscent, which means it's very easy indeed to inspect them - if they're failing it's easy to see because it's not covered by an opaque coating. You then just brush that area and reapply.

I've found that a single good coating lasts for around a couple of years on bare metal, out in all weathers including salty winters.


They're around £12 a can each on ebay (from Rexel), typically 3 - 4 cans will do the complete underside of an average small / medium car.

Richard53

Rub the area back to bare metal, or at least get rid of the loose stuff. Use a rust killer like Kurust or Jenolite. This converts the rust to a hard black substance (not sure of the exact chemistry, but it works). I have no experience of stone chip coatings, but I can vouch for the effectiveness of Waxoyl, applied with a brush. Like Joe Horner's Dinitrol (which I haven't used) it will need checking/renewing every year, but it's only half an hour's work and it's relatively cheap.

I have a small trailer with a steel frame that lives outside. I treated the frame as above (it was already quite rusty) and it was a good 5 years before rust started to show through again.

forgotmyname

Rust killer and a zinc rich primer then a protective coat of paint.