how to dig rust out of a ferro boat?

Hi guys i have a 1974 ferro yacht 39ft. it has occured surface rust on the hull roughly 6inch's diameter wide. trying to find out steps on how to repair this and what tools and product to use? not sure how far down will have to go to fix this yet. any help would be appreciated.

 

Is this below the waterline, just stained or actual rust from metal close to the surface and is the cement cracked?

 

below the water line, concrete not cracked, actual rust seems to be sepping through, will take it out of water in a month for diy repair. assuming i will have to slowly chisel away at the rust untill no more?

 

also same problem on top deck under floor boards, asuming will have to rip small section of floor boards up and chisel away at rust?

 

Are there any indications of weakening of the structure?

 

Concrete is not waterproof, rebar rusts eventually. It expands when it does. Nothing good can come of it. This is the downside to stone boats.

Search for a ferrocement forum. This would be a topic much discussed and you could greatly benefit from the real world experiments of countless others.

 

A small amount of rust generates a lot of stain.

Use a compressed air driven needle gun otherwise known as a descaler. A good tool fires the pins much more effectively than the cheap chinese tools.

Needle gun the cement off the steel and expose the outer layers of mesh back to the rebar. keep chasing it back till the bar is clean. You can also de-scale the rebar with the same tool. rusty mesh should be re-patched.

Epoxy the raw mortar faces and then immediately re-plaster with a correct mix with pozolanic additive.

Good mortar is remarkably impervious to water and the steel will not rust within the layup unless cracking occurs. ie it rusts becasue of the crack , at that point it will expand, but otherwise it doesn't rust as evidenced by numerous old ferro vessels.

 

patch it up pretty and sell it .... quick .....

 

I agree with Mike Johns, a good ferro built in approved materials is very tough and impervious and gets harder with age but if the steel mesh is a bit too close to the surface as can happen in places then any slight crack or pinhole that may be caused by an impact or insufficient covering will cause a little local rusting that looks bigger than it actually is, so this may even just turn out to be a small cosmetic repair.

My boat has the odd little rust stain under the paint inside the hull where the mesh is more likely to be near the surface but it has never got any worse and if it did I'd just smooth it off with the angle grinder. It would have to be a really obvious crack to stand any chance of penetrating further but ferro is very easily repaired anyway so don't worry.

 

There you go, things perhaps are more optimistic than I thought. Needle guns and ear plugs are a great companions. Mine is of the cheap chinese pursuasion, a higher quality one would be an improvement. Best of luck.