Phosphoric acid

[muzaz]

I'm doing some bulkhead and chassis repairs at the moment, cut out the rotten areas but would like to treat the areas with surface rust and not through rust. I normally use Wurth rust converter but for external exposed areas it seems to rust again after a while. Probably the conversion does not go deep enough.

So came across a product which is basically phosphoric acid with some additives and had a go with it. I was thinking the surface would go black but instead it turned white! I diluted the acid to about 50%, when applied I could see some fizzling on the surface so something was going on. I left the acid there to dry up as I read somewhere that if you wash it off it will flash rust. 

Any idea of how a correctly treated surface should look like?

[Ed Poore]

Dad used phosphoric acid to treat patches of rust on his shogun. I tend to take a wire wheel, brush, whatever I can get in and get rid of the rust and paint it.

He's on his 3rd or 4th Shogun in the time I've had my 110 and my chassis (bar cross member) is still original.

I'll just leave that there... 

[Stellaghost]

30 minutes ago, muzaz said:

I'm doing some bulkhead and chassis repairs at the moment, cut out the rotten areas but would like to treat the areas with surface rust and not through rust. I normally use Wurth rust converter but for external exposed areas it seems to rust again after a while. Probably the conversion does not go deep enough.

So came across a product which is basically phosphoric acid with some additives and had a go with it. I was thinking the surface would go black but instead it turned white! I diluted the acid to about 50%, when applied I could see some fizzling on the surface so something was going on. I left the acid there to dry up as I read somewhere that if you wash it off it will flash rust. 

Any idea of how a correctly treated surface should look like?

I believe @Gazzar has done this on his 101 thread in members vehicles, he may be able to advise

Regards Stephen

[Gazzar]

I've used straight phosphoric acid. Brush on. It dries white, remove all residue, wirebrush or whatever, and you are left with an almost galv surface. If there's still rust, repeat. 

Basically, the cleaner, the better. Then 2k zinc phosphate on top. Ensuring the surface is well keyed, coarse sandpaper works for me.

[Gazzar]

It's just another part of the system:

Remove, treat, prevent, isolate and protect.

Blast, acid, zinc, epoxy, top coat.

[muzaz]

7 minutes ago, Gazzar said:

I've used straight phosphoric acid. Brush on. It dries white, remove all residue, wirebrush or whatever, and you are left with an almost galv surface. If there's still rust, repeat. 

Basically, the cleaner, the better. Then 2k zinc phosphate on top. Ensuring the surface is well keyed, coarse sandpaper works for me.

Good so the white stuff is a good sign, will continue the process as suggested.

Have another question regarding heavily corroded areas where I cannot get to bare metal. I'm repairing the dumb irons, cut the rotten part off but I can still see corrosion inside the chassis rail. It's not rotten but there is quite a bit of rust and I can't reach that with a wire brush.

I applied the acid, some parts turned white but most are dark brown/black. Will the phosphoric sort of block the corrosion?

[Gazzar]

Don't think so, internal non structural rot is probably best treated with a creeping cavity wax. Many superb products out there: dinatrol, built hambler, etc. 

 

Don't use old engine oil, the drips cause motorbikes to crash. Waxoil is disappointing.

[Ed Poore]

11 hours ago, muzaz said:

Have another question regarding heavily corroded areas where I cannot get to bare metal. I'm repairing the dumb irons, cut the rotten part off but I can still see corrosion inside the chassis rail. It's not rotten but there is quite a bit of rust and I can't reach that with a wire brush.

Probably worth getting or borrowing one of those drain cleaning attachments for a pressure washer. Before you weld up the end of the chassis shove it in and blast out what carp you can. My pressure washer (my parents old one and repaired and got working again) punched a hole through the old FIAT's door my grandparents gave them. It wasn't even that bad rust wise.

You might be able to knock off any loose rust that way and clean it out before adding your chosen treatment. Also worth ensuring the drain holes are nice and clear. It's the one thing I tend to do everytime I go under the vehicle is clear them out with a screwdriver.

Lack of paint and general neglect on the chassis has actually kept mine in pretty good condition but then it gets used regularly so on/off road grime gets washed off fairly regularly. Waxoyled it once and pressure washed it half a dozen times in 15 years of ownership and it's still (bar rear cross member and bits of the front dumb irons) original. When the cross member was built the guy who made it and welded it on was a Polish ex-mechanic he gave the inside of the chassis a thorough inspection and said it was in amazing condition especially considering it's age.

[FridgeFreezer]

15 hours ago, muzaz said:

I normally use Wurth rust converter but for external exposed areas it seems to rust again after a while

I thought with any & all of these treatments you always have to at least paint over the treated surface to protect it afterwards, I'm not aware of any that are "treat & forget"?