Sand blast or wire brush on angle grinder?

[peterf]

I want to take my keel back to bear metal and prime with many coats of primacon. I've used a wire brush on an angle grinder to do this before, it works but no fun.
I'm curious whether I should try purchasing an add-on for my Karcher to do sandblasting instead?
Has anyone tried it and how many bags of 25kg sand would I need for a keel on a gibsea 312?

 

[SailingDog]

Don't know the answer to your sand question. Is there anybody in your yard having their hull blasted ? May be a good price could be agreed as they have the blasting kit and staff already on site, it's an idea.

 

[BarrytheViking]

I found using flap discs on an angle grinder relatively easy and much better than a wire brush.

 

[Ammonite]

Forget the Karcher. You will be there forever and use a ridiculous amount of sand. You will also need a load of replacement knozzles as the sand eats through them. Ultimately it works but is incredibly slow and I wouldnt tackle anything large. I used it to remove 20 years of antifoul on a 28ft long keel where the keel joins the hull and it was ideal for that but that was a long strip about 3" wide. I used a carbide scraper for the rest which works a lot better but is a horrible job. Get it professionally blasted instead!

Dont hold me to it but from memory a 25kg bag will do less than a square foot it theres a decent build up of paint.

Im also not a fan of any form of dry sanding / grinding. You may have the correct ppe but others nearby wont

 

[penberth3]

I found using flap discs on an angle grinder relatively easy and much better than a wire brush.

Agreed, a wire brush can leave a layer of oxide. Looks like bright metal but it's polished rust.

 

[WoodyP]

My diy attempt at sand blasting proved to me that you need a professional machine. It rusts almost as soon as you stop blasting. The best diy cure was getting as much scale off as possible then a couple of coats of fertan before going in with the primocon. Then repeat annually where the rust comes through. I find that you don't notice a bit of rust if you use red antifoul and get it back in the water.
Have also tried the Dulux metal paint which wasn't any better or worse.
I got fed up with repainting coat after coat of primocon. I wish there was something better.

 

[peterf]

In the time taken to check on home schooling progress and make a cup of coffee I have 6 wonderful answers.. I'll order some flappy discs and give that a go. Many thanks for saving me the hassle of experimenting with sandblasting.

 

[Tranona]

Suggest you use this rather than Primocon. Around half the price and just as good.
smlmarinepaints.co.uk/primers/single_pack/vinyguard

 

[Crazy-Diamond]

I was very impressed with the flap discs when I took my keel back to the metal . They are not expensive, so buy quite a few so you're not afraid of changing to a fresh one, or even running out. Having a few spare is never a bad thing, but persevering with knackered ones is not rewarding!

 

[dankilb]

I used these on our keel - they're composite so no risk of grinding metal into the metal: Non-Woven Preparation Wheel 115mm

Found they would do about a m2 before wearing out. Not bad for £5, but not great. Flap discs are also good. Probably get through them at about the same rate.

 

[BobsFolly]

I've used the Tercoo rotating blaster to good effect, albeit not on the full keel.

Tercoo®

PBO did a product review and were fans too.
Attempting to attach a poor quality but hopefully legible screenshot!

Bob

 

[MikeCC]

I found the Tercoo great for getting the layers of paint off. It's not super quick but easily lasts the distance. Finished off with the polycarbide disks on a grinder. They are effective but wear quickly.

Might be worth looking at an industrial zinc-based epoxy primer on to the metal. Some of them are intended for seawater or salt-spray environments. Couple of coats before fairing then 2-3 coats marine epoxy primer. I used that approach and only got a couple of spots coming through (where I think there were some small voids).

 

[dankilb]

Might be worth looking at an industrial zinc-based epoxy primer on to the metal. Some of them are intended for seawater or salt-spray environments. Couple of coats before fairing then 2-3 coats marine epoxy primer. I used that approach and only got a couple of spots coming through (where I think there were some small voids).

Absolutely! This is the way to go if you don't want the rust to return (some just accept it does/will and grind/treat as required each season). Vinyl primers like Primocon or Vinyguard only offer modest moisture resistance...

I used as many coats of Jotun Penguard Express ZP (the ZP being zing phosphate) on our keel as it took to finish up the 5l tin! I can't praise the stuff highly-enough. Super easy to use and sticks like the proverbial. It even set up rock hard in various used roller tray liners/mixing pots I left sitting about in the elements after use.

 

[rogerthebodger]

Having a steel boat the only real way is to grit blast (not sand as its too soft)

The next alternative is needle descale and /or chipping to remove the major rust buildup.

I then follow with hydrochloric acid until the metal is a uniform gray colour with no black oxide showing.

The wash with phosphoric acid to convert any remaining oxide followed by a wash with fresh water and blow dry.

The paint with epoxy tar at least 6 coats overcoating within 48 hours.

 

[Neeves]

For grit blasting I suspect Roger might refer to using carborundum, which is as hard as you can get.

Galvanisers need clean steel and if grit blasting on site, grit blast and then galvaniser, no delay - or the steel corrodes and the galvanising does not take. The alternative is an acid wash which is what Roger is suggesting, then blow dry (or work in Sydney in the sun with low humidity ) - other wise the steel rusts very quickly - as WoodyP said. It might be the humidity is too high in the UK to expect steel not to rust quickly.

Jonathan

 

[rogerthebodger]

For grit blasting I suspect Roger might refer to using carborundum, which is as hard as you can get.

Galvanisers need clean steel and if grit blasting on site, grit blast and then galvaniser, no delay - or the steel corrodes and the galvanising does not take. The alternative is an acid wash which is what Roger is suggesting, then blow dry (or work in Sydney in the sun with low humidity ) - other wise the steel rusts very quickly - as WoodyP said. It might be the humidity is too high in the UK to expect steel not to rust quickly.

Jonathan

I used slag from a refining process but Carborundum would work

galvanizer pretreat any steel with a dip in acid before a wash and dip in zinc so you only need to blast before galvanizing to remove any paint as I did when I galvanized my stern hatch.

Also when treating rust on a painted surface you must remove the paint around the rust otherwise the will be rust under the paint that will re appear.

When I grit blasted my boat when building my boat was 6000 ft above sea level and 600 km from the sea in winter (clear blue skies) so having no rain for a good 3 months so the lowest humidity possible

 

[dankilb]

I was amazed by how effective a simple wipe down with dilute phospohic acid was, after grinding our keel. In the end (and another story...!), we had the keel blasted anyway, but the blasting technician commented on how well the acid had penetrated the cast iron. It was actually harder for him to see where he'd blasted the surface clean, because of this (the acid left a black colour - whereas the iron itself blasted back to a grey colour).

 

[Neeves]

Some keels on some yachts, commonly steel yachts, are constructed from HT steel plate. They may have ballast, within the keel, as a torpedo at the bottom of a HT fin and sometimes the hollow, or part hollow keel is used to store water or fuel. Acid treatment of HT steels, can (I stress the can) lead to hydrogen embrittlement and for this reason the galvanising of HT steels is conducted without acid treatment. The only option is grit blasting - but you need to grit blast and galvanise immediately after. (or the freshly exposed steel starts to rust). If the galvaniser does not grit blast in house you need to coordinate the 2 services. Manson (of Supreme fame - you can get anchors into almost any thread ) in NZ offer this service, and build hollow keels. I believe Manson fabricate components (including anchors), in house and have their own galvanising bath. Wild Oats XI (of Sydney/Hobart fame) has a Bisalloy HT steel fin.

Different issue but in Tasmania there is a lot of tannin in the water, including seawater, and you can identify yachts that have spent time there, even only a few weeks, as they have an unattractive brown stain above the water line. It is very common to wash your yacht, AWB, with an acid wash to remove the stain (when you slip for AF) - but I think they use Oxalic.

And try not to paint your anchor chain, even the length/depth marks, as its devil to remove if you want to re-galvanise.

Apologies to the OP - criminal thread drift here. . My fault entirely

Jonathan