Rust Prevention

[Defender110]

What I've done with any welded repairs is to coat the whole patch, not just the seams, with polyurethane sealant. Then the whole lot with underbody wax. !

That's my trick too! works a treat.

[BX Bandit]

What be a polyurethane sealant?

[mat_fenwick]

Something like Tigerseal, or Sikaflex 521 for example (although I've not been too impressed with the former). Some types of silicone sealant (the ones which smell like vinegar) will cause the steel to rust beneath it.

[Defender110]

http://www.wisegeek.org/what-is-polyure ... ealant.htm" onclick="window.open(this.href);return false;

[citronut]

i belive you can coat it in anything that is not porous, as you just need to stop the air getting at the bare metal inside,outside and between any joints/seams,

regards malcolm

[BX Bandit]

Something like Tigerseal, or Sikaflex 521 for example (although I've not been too impressed with the former). Some types of silicone sealant (the ones which smell like vinegar) will cause the steel to rust beneath it.

Is brushable seam sealer suitable? Where I've applied it, it seems really durable yet like a firm rubber.

[barry badger]

IMO seam sealers the best as long as it's not being applied to too great a depth and it's all owed to dry out thoroughly before overcoating. I've tried several polyurethane adhesives and not been overly impressed with their surface adhesion.

[Defender110]

I've tried several polyurethane adhesives and not been overly impressed with their surface adhesion.

I've found quite the opposite, we build wagon/generator bodies with it and stick 8' x 4' aluminium sheets on using it and they do not come off, you have to use a stanley blade to slice through the sealer when you need to replace them. Sticke like poop to a blanket.
Brushable seam sealer is just as good.

[BX Bandit]

Seam sealer it is then - that'll do me.

Where it comes to sealing spot welded joints, surely there is a chance of sealing one edge, whilst allowing water in at the other edge? So providing a lovely trap for rust to get started?

[mat_fenwick]

That's where cavity wax comes in!

[BX Bandit]

I'd still be concerned though that water would get in and not out!

[mat_fenwick]

Well yes, it will tend to over time but by regularly applying wax into box sections you can minimise the chances. If you could do a perfect job with everlasting waxes and sealants it would never rust, but all coatings will fail in the end. Which is where galvanising paints and zinc rich primers come into play (or ideally proper galvanising!), as the zinc content will corrode before the steel. This should give you a fighting chance to apply a top coating again before the rust starts.

[BX Bandit]

You bring me to my next question actually: why not use cold galv (due to high(est) zinc content) as opposed to a zinc rich primer?

[mat_fenwick]

The 3 cold galvanising coatings I know of are Zinga (96% zinc), Galvafroid (90-92%) and (Bilt Hamber) Electrox (90+%). Over the last few years I reckon Zinga beats Galvafroid slightly for corrosion protection, dunno about Electrox yet but it seems to contain noticeably less zinc based on how it sands - the other 2 tend to smear a bit as you expose the metal particles. Electrox seems to sand more like a standard zinc rich primer, or for that matter like normal paints.

Zinc rich primers I'm guessing contain less zinc (they don't tend to say what content but the above 3 feel much heavier than paint) so will give less protection. I would guess significantly less protection than just a sliding scale based on zinc content, as if it's 90+% zinc the particles will likely be in electrical contact with each other. So the effective surface area that will be in contact with any bare steel (where the coating is broken) will be much higher and will take correspondingly longer to be preferentially corroded away before the steel starts rusting.

Hope I've explained myself well enough! Bit of a comparison here but obviously biased towards Zinga...

Disadvantage of cold galv. is that it seems less adherent to smooth steel - certainly the instructions I've seen give a desired roughness finish for the steel. So I'm guessing a primer will stick better to a smooth panel but not done a back to back trial to compare.

On the subject of back to back testing, I keep meaning to do something with cavity/underbody waxes. I was previously of the opinion that they were all the same, but I saw an interesting experiment on another forum (will see if I can find the link) which persuaded me otherwise. Would be good to check with my own eyes as I'm naturally suspicious (plus he only tested BH stuff versus Waxoyl; Dinitrol might be worth a look).

[BX Bandit]

That makes perfect sense mate and that Zinga stuff looks good. Just pulled up their website so I'm going to browse.
On the subject of testing different waxes, I'm going to try a comparable test but not in any cavity (which I admit would be a better test).
I shall try to find an area of rust big enough to apply two different waxes side by side and aside from the application of the wax itself, no other treatment will be applied. Leave for one year and then see how the rust has fared.

I needed a shutz can to pour the BH cavity wax stuff into so I bought the cheapest product from eBay which was a British made alternative to waxoyl, made by a company called Silver Hook