Jump to content
Strawberry Orange Banana Lime Leaf Slate Sky Blueberry Grape Watermelon Chocolate Marble
Strawberry Orange Banana Lime Leaf Slate Sky Blueberry Grape Watermelon Chocolate Marble
Ballyeli

Angle Grinder for sanding on hull

Featured Posts

I recently bought a 40ft narrow boat steel hull,

After getting it lifted and cleaned for a survey I discovered quite a lot of rust and a seam in the middle of the boat which needed filling and some pitting towards the front.

I got onto a welder and to save some time and money was hoping ( in favour of sand blasting) to use a grinder with a flat plate to sand down hull and prep it for welding.

Grinder is a dewalt battery 8000rpm and 180grit plate.....am I in any danger of eating through the steel and grinding it down too thin?

IMG-20200914-WA0018.jpeg

Share this post


Link to post
Share on other sites

 

Use cup brushes, don't mess about with a sanding disc.  Just don't go too big or you won't be able to control it.

  • Greenie 1

Share this post


Link to post
Share on other sites

I was just gonna suggest using a cup brush. 
I don’t think you can do any damage with one. 

69115660-40E7-4A9B-9B63-8CB39C4D0D6E.jpeg

  • Greenie 2

Share this post


Link to post
Share on other sites

If using a battery grinder you will need quite a few batteries. I have the Makita and if working the wire cup brush quite hard doubt I would get more than 5 to 10 minutes out of a 3amp-hour battery.

 

..............Dave

Share this post


Link to post
Share on other sites

Or use wire wheels. They're a bit more powerful than cup brushes. I've bought some good ones quite cheaply on eBay. 

 

Be sure to wear eye protection, ear defenders and leather gloves. 

 

image.png.5be174cf41245cdc1e89bad08dec5b4d.png

Don't use sanding discs or flap wheel discs below the waterline. They take quite a bit of metal off along with the paint. And don't listen to the people who tell you that wire wheels polish the steel so much that paint won't stick to it. Those people are rank amateurs and don't know what they're talking about! 😂

1 minute ago, dmr said:

If using a battery grinder you will need quite a few batteries. I have the Makita and if working the wire cup brush quite hard doubt I would get more than 5 to 10 minutes out of a 3amp-hour battery.

 

..............Dave

 

Yes I think a battery grinder is going to be a to waste of time for more than a small area of paint removal.

Edited by blackrose

Share this post


Link to post
Share on other sites

just to remind folks that you can buy sanding machines that look like angle grinders but rotate at 3,000 rpm.  these are available in 9" and 5" versions.   much safer and can be used for polishing and buffing if needed.

  • Greenie 1

Share this post


Link to post
Share on other sites
9 hours ago, blackrose said:

 And don't listen to the people who tell you that wire wheels polish the steel so much that paint won't stick to it. Those people are rank amateurs and don't know what they're talking about! 😂

 

 

So that includes Blakes Paints (now Hemple) then. That is exactly what they advised in a booklet I once has and said if using brushes follow this up with a light disking to key the  surface. This was in respect of blacking.

 

So the choice is the OP's follow your advice or spend an extra hour and follow Blakes, i know which I would do.

 

 

Edited by Tony Brooks
  • Greenie 1

Share this post


Link to post
Share on other sites

 

Don't know what other people may think of this, but I'd be inclined to fill some of the pitting, after fettling the steel, with a marine epoxy filler and apply an epoxy paint over the work, before eventually applying a blacking. 

 

 

Edited by Higgs

Share this post


Link to post
Share on other sites

I haven't got a handy electron microscope to see what the surface of steel looks like after attacking it with wire wheels but a wire brush can make a surface shiny and a sanding disk can make it matt so I think I will carry on with a big heap of sanding discs to key the steel after more aggressive treatment to shift rust, scale and old paint, Sanding discs clog too quickly for the initial treatment and if there are traces of bitumen they can melt the old covering and make a right mess.

Share this post


Link to post
Share on other sites
8 minutes ago, Higgs said:

 

Don't know what other people may think of this, but I'd be inclined to fill some of the pitting, after fettling the steel, with a marine epoxy filler and apply an epoxy paint over the work, before eventually applying a blacking. 

 

 

Other people might think that's a silly idea but I think its a really good plan, after all GRP boats (with various chemistry) use that sot of stuff to keep the thing afloat.

Share this post


Link to post
Share on other sites
4 minutes ago, Bee said:

Other people might think that's a silly idea but I think its a really good plan, after all GRP boats (with various chemistry) use that sot of stuff to keep the thing afloat.

 

Thanks for the thumbs up.  Each time I black the boat, I scrape back any developing pitting in the blacking. Pits are a trap for water and may not allow the surface to have dried thoroughly after jetwash, before the new blacking goes on. Filling will flatten some of the pitting and help avoid these cavities forming. The boat will also look better. 

 

 

Share this post


Link to post
Share on other sites
1 minute ago, David Mack said:

Sanding discs won't clear corrosion out of pits like a wire brush will.

You're right, Its the final bit of prep before painting that a quick (!) flick over with the sander that I'm really thinking of  and also, a slightly different situation, epoxy paint is hard stuff, I,ve never used a wire disc on mine but a wire brush seems to polish it. Hard to tell really, Paint gets tested in laboratories then used by some lazy so and so in a cold wet dock.

Share this post


Link to post
Share on other sites
1 hour ago, Murflynn said:

just to remind folks that you can buy sanding machines that look like angle grinders but rotate at 3,000 rpm.  these are available in 9" and 5" versions.   much safer and can be used for polishing and buffing if needed.

^^^^^ This.  Greenie given.  They're also available in 6" (I have one).

Share this post


Link to post
Share on other sites
4 hours ago, Bee said:

I haven't got a handy electron microscope to see what the surface of steel looks like after attacking it with wire wheels but a wire brush can make a surface shiny and a sanding disk can make it matt so I think I will carry on with a big heap of sanding discs to key the steel after more aggressive treatment to shift rust, scale and old paint, Sanding discs clog too quickly for the initial treatment and if there are traces of bitumen they can melt the old covering and make a right mess.

That is exactly what Blakes (Hemple) recommended.

Share this post


Link to post
Share on other sites
5 hours ago, Bee said:

Sanding discs clog too quickly for the initial treatment and if there are traces of bitumen they can melt the old covering and make a right mess.

Worth pointing out at this juncture that the reason (some) sanders are equipped with variable speed is so that you can select a slow speed such that they won’t heat up and melt the paint you’re attempting to sand. Won’t help an awful lot with already-soft bitumen but it’s worth remembering when keying paint. 

Share this post


Link to post
Share on other sites
17 hours ago, Goliath said:

I was just gonna suggest using a cup brush. 
I don’t think you can do any damage with one. 

69115660-40E7-4A9B-9B63-8CB39C4D0D6E.jpeg

Just to add a little to this good advice, don't buy anything larger than 65mm as they are often badly balanced and will destroy both the machine and your hands due to vibration. Discard when the first bristles start to shed or you will resemble a chrome plated hedgehog, and some angle grinders have a rotational speed in excess of that recommended for the cup brushes which will also lead to early faliure. 

https://www.ebay.co.uk/itm/283052817968

 

These are a good buy.

Share this post


Link to post
Share on other sites
7 hours ago, Tony Brooks said:

 

So that includes Blakes Paints (now Hemple) then. That is exactly what they advised in a booklet I once has and said if using brushes follow this up with a light disking to key the  surface. This was in respect of blacking.

 

So the choice is the OP's follow your advice or spend an extra hour and follow Blakes, i know which I would do.

 

 

 

But if grit blasting isn't available Jotun recommend prep with power tools with wire wheels without any sanding.

 

So the choice is between following Blakes advice (and taking off good steel in the process), or following Jotun's advice. I know which I would do, which is also what I've always done without any problem.

 

Everyone is free to decide for themselves of course.

 

 

Share this post


Link to post
Share on other sites
31 minutes ago, blackrose said:

 

But if grit blasting isn't available Jotun recommend prep with power tools with wire wheels without any sanding.

 

So the choice is between following Blakes advice (and taking off good steel in the process), or following Jotun's advice. I know which I would do, which is also what I've always done without any problem.

 

Everyone is free to decide for themselves of course.

 

 

So how much good steel is taken off with a light sanding to give a key after dealing with the rust and pits with a wire brush?

 

I doubt its more the 5 thou (0.127 mm) and that is only in the base of the scratches.

Share this post


Link to post
Share on other sites
1 hour ago, Tony Brooks said:

So how much good steel is taken off with a light sanding to give a key after dealing with the rust and pits with a wire brush?

 

I doubt its more the 5 thou (0.127 mm) and that is only in the base of the scratches.

 

"A light sanding" is subjective. How much steel ends up being removed really depends on the skill of the operator and the grit and quality of the paper being used. If it's done every couple of years then after 10 years an unskilled operator like the OP who possibly doesn't have much experience, might have removed quite a lot. Even your minimal 0.127mm X 5 = 0.63mm. Suddenly it doesn't seem quite so insignificant!

 

I know that I've easily removed half a mm of good steel above the waterline in one go with a disc sander without even trying. You definitely don't want to do that below the waterline in the hull.

 

I know what I will and won't do, but as I already said, people are free to do whatever they want with their own boats. 

Edited by blackrose

Share this post


Link to post
Share on other sites
On 16/09/2020 at 22:18, TheBiscuits said:

 

Use cup brushes, don't mess about with a sanding disc.  Just don't go too big or you won't be able to control it.

Yes absolutely!! I really wish I read this before I went at it guys had about 20mins of battery life, had a small drill brush and a scotch brite disc for grinder. Got the seam somewhat clean but there's some pitting I have to find so I'm going at it again with 4 more batteries Saturday!

IMG-20200917-WA0014.jpeg

Share this post


Link to post
Share on other sites

Not wishing to spoil your fun but it looks from the picture like this boat has been in salt water and there are holes in the plating. Now it has dried out try hitting as small area with  chipping hammer to see if the scale falls off and what you are left with. Anything less than 3-4mm thick and you have a serious problem. I don't think a wire brush is going to have much impact on that scale except for polishing it. You need an air chisel, needle gun or impact drill with a chisel bit or shot/sand blasting

Good luck with it and you will learn from your experiences.

Share this post


Link to post
Share on other sites

Quite. That needs proper cleaning, and you're not going to achieve that with cordless tools. If there's no mains on site you need a generator.

Share this post


Link to post
Share on other sites
1 hour ago, Ballyeli said:

I'm going at it again with 4 more batteries Saturday!

A cheap mains grinder and the hire of a genny would cost less than 4 decent batteries. And if you have no mains how are you going to charge them?

Share this post


Link to post
Share on other sites

8mm think originally and about 5.2 in parts now 3.2 at seam don't get me wrong realistically replating is probably the way forward but I've about a €2k budget to get this painted and back in water.  Worry about the proper job to be done after the Covid budget!

Batteries on loan from various friends but not wrong about charging hoping to get more than an hours work in!

Got 2 of these little bad boys out of the motor factors when getting the cup brush 

Screenshot_20200918-123145.jpg

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.