Technical What paint for an exhaust piping?

[(CZ)enda]

This is something to do when the spring comes and the winter damages are to be fixed on my Mk1 45.
The piping in question is that from manifold to muffler. It shows the signs of deep corrosion from the outside.
BBQ Hammerite, although heavily advertised, is out of question (according to independent tests, it is too brittle and not enough heat-resistant).
Are there any other (proven) choices available not exceeding 1/2 of the price of a new aftermarket piping?

 

[Chas Uno]

This is something to do when the spring comes and the winter damages are to be fixed on my Mk1 45.
The piping in question is that from manifold to muffler. It shows the signs of deep corrosion from the outside.
BBQ Hammerite, although heavily advertised, is out of question (according to independent tests, it is too brittle and not enough heat-resistant).
Are there any other (proven) choices available not exceeding 1/2 of the price of a new aftermarket piping?

There is paint that I've used in the past that is generically known as 'VHT' paint (Very High Temperature). It is used for exhausts and manifolds, or any metal that runs at extremely high temperatures.

The downside with exhausts though, is that most are made of mild steel. Exhaust gases contain a lot of water and corrosive chemicals, so they tend to rot from the inside out. Mild steel is not very corrosion proof, which is why exhausts tend to rot through every few years :-(

I've used VHT paint with mixed success. I found that manifolds responded better than exhaust tubing! Probably because they are made of thicker metal/ cast iron so are less likely to rot through from the inside out.

You can also have manifolds and exhausts sprayed in aluminium (I think) which is expensive, though looks good and helps keep corrosion at bay.

The best way to stop an exhaust and manifold from rusting is to replace them with stainless steel items. But as always there is a downside! Stainless steel is more brittle than mild steel so can fracture more easily.

Your cheapest option is to ignore it and replace it every few years!

 

[alexGS]

I agree with everything that Chas said (great summary!)

I've found that zinc-rich primer (CRC Cold Galv or something) worked - up to a point. It cures off OK but still seems prone to flaking.

If the pipe is already pitted from the outside, I'd say don't bother - just wait for it to fall apart and then pay for a mild-steel replacement. If you were planning to keep a car for ages (say, >6 years) then stainless steel would be a fair choice - otherwise, mild steel will be cheaper (some exhausts I know of in the UK have lasted for 8 years - in our climate, well... my Alfa still has its original exhaust after 13 years.) CZenda, I know you don't live in the UK, but I thought I'd mention it for comparison

With your new exhaust, try the VHT paint that Chas used (maybe the aluminium colour) or the CRC zinc-primer that I used - it may add a little to the life, but the benefits will be mainly cosmetic since the rust will be inside-out.

The welded area is a slightly different story - I believe that painting welds extends the life a little, because the steel around the welds is more crystalline (more corrosion-prone), and the heat of the welding tends to burn off the protection that the pipe originally has. But we're only talking about a year or two extra, tops, so instead of five years you may get six... you get the idea

Manifolds are always the ugly point of any engine bay, so with your leftover high-temp silver paint, you can spray the manifold some day. The result is always better if you remove it first, and wire-brush (ideally, paint on some rust-convertor of the acid type first. Deox-C is the current pick of the products in the UK, to my knowledge.)

-Alex