Two Pack Epoxy Blacking Advice

[Gellscom]

Hi

 

i am on a fact finding mission, according to records and our survey our boat was shot blasted and  blacked with a two pack epoxy. As of writing this the exact date of this is unclear but we believe within the last five years. It seems well adhered and generally in good condition all be it aging a little.
 

We would like to do the next blacking with epoxy again but I can’t seem to clarify the preparation needed. On videos I have seen jet-washing when removed from the water clears the growth but can leave a white residue that I believe is lime scale. So my questions are :

 

Do we have to shotblast the boat again to black a second time with two pack epoxy?

If not what additional preservation is needed to remove the lime scale ?

Any recommendations for the best epoxy products to use.

 

thanks in advance for any advice tended.

 

[BEngo]

No need to  shotblast unless the epoxy is in really poor condition with rust coming through.

 

Preps for overcoating with epoxy:

 Jet wash.  This should really be powerful enough to get the limescale off.  The Circus Field machine does, at 3200 psi, though the right nozzle helps.

 

Then abrade with a wire brush in an angle grinder,   making sure all the remaining  limescale is removed.  You can use either a twist knot or a crimped wire brush. I find I can do the sides with a 9in  grinder but need a small one for the bottom and fiddly bits. Don't  forget the rudder tube and the insides of the weed hatch.  No need to go mad.  You just need to remove the oxidized top layer of epoxy so it goes back to black from greyish.

 

Clean any damaged areas of coating to bare metal with abrasive discs. 

Patch epoxy prime or epoxy the disced areas. 

 

Apply one or two coats of epoxy. Follow the application data sheet carefully for temperatures, humidity etc.

 

Prep is harder work than many are used to, so allow at least a week out of the water and don't plan to be partying much.

 

N

[Midnight]

2 hours ago, Gellscom said:

 

We would like to do the next blacking with epoxy again but I can’t seem to clarify the preparation needed.

 

 

I've used two-pack from new, re-blacking every two to three years. After jet-washing I use an angle grinder with a wire cup wheel to scruff up the existing coating then add two coats. I've not had any issues.  Because of changes in environmental issues affecting manufacturers/suppliers I've had to change types and brands twice. I now use Hempel Hempadur 45143 which I was told would lift the original 15130 (no longer available) but so far after 6 years it's been okay. I'm due in drydock in March but not expecting any issues.

[system 4-50]

Are you sure its limescale?  Some epoxy goes grey with time.

[blackrose]

3 hours ago, BEngo said:

No need to  shotblast unless the epoxy is in really poor condition with rust coming through.

 

Preps for overcoating with epoxy:

 Jet wash.  This should really be powerful enough to get the limescale off.  The Circus Field machine does, at 3200 psi, though the right nozzle helps.

 

Then abrade with a wire brush in an angle grinder,   making sure all the remaining  limescale is removed.  You can use either a twist knot or a crimped wire brush. I find I can do the sides with a 9in  grinder but need a small one for the bottom and fiddly bits. Don't  forget the rudder tube and the insides of the weed hatch.  No need to go mad.  You just need to remove the oxidized top layer of epoxy so it goes back to black from greyish.

 

Clean any damaged areas of coating to bare metal with abrasive discs. 

Patch epoxy prime or epoxy the disced areas. 

 

 

Good advice apart from the bits I've highlighted in bold. I think you'll go straight through good epoxy with a wire wheel on an angle grinder. It's too aggressive. All you want to do is key the surface so you should use a medium grit paper on an orbital sander and lightly key the surface.

 

For any bare metal or rust that's when I'd use the wire wheel/angle grinder. If you use abrasive discs you'll be taking off good steel, so you don't want to do that particularly on hull areas below the waterline. Again, it's too aggressive. 

 

Just my personal opinions but also backed up by head of technical at Jotun who I've spoken to a couple of times.

 

 

16 minutes ago, system 4-50 said:

Are you sure its limescale?  Some epoxy goes grey with time.

 

Yes, chalking. Quite normal and doesn't affect the physical properties of the paint.

[BEngo]

2 hours ago, blackrose said:

I think you'll go straight through good epoxy with a wire wheel on an angle grinder

Then you are pressing too hard. You really only need to let the brush skate over the surface and take the chalking off.  The brush will not clog like abrasive paper.

 

On the other hand, a wire brush will not remove all the rust.   An abrasive disc will.  Again, I said don't go mad; use something like a  120 grit, aor finer, disc after pressure wash and dry off.  All the wet rust will have gone but you may have a few patches above the waterline from mechanical damage etc.  These are not easily removed by a pressure washer.    There is no need to remove measurable quantities of metal.  If there are still  pits then you need rust treatment,  localised blasting  or an all over blast depending on the number and depth. 

N

 

  

[LadyG]

8 hours ago, BEngo said:

No need to  shotblast unless the epoxy is in really poor condition with rust coming through.

 

Preps for overcoating with epoxy:

 Jet wash.  This should really be powerful enough to get the limescale off.  The Circus Field machine does, at 3200 psi, though the right nozzle helps.

 

Then abrade with a wire brush in an angle grinder,   making sure all the remaining  limescale is removed.  You can use either a twist knot or a crimped wire brush. I find I can do the sides with a 9in  grinder but need a small one for the bottom and fiddly bits. Don't  forget the rudder tube and the insides of the weed hatch.  No need to go mad.  You just need to remove the oxidized top layer of epoxy so it goes back to black from greyish.

 

Clean any damaged areas of coating to bare metal with abrasive discs. 

Patch epoxy prime or epoxy the disced areas. 

 

Apply one or two coats of epoxy. Follow the application data sheet carefully for temperatures, humidity etc.

 

Prep is harder work than many are used to, so allow at least a week out of the water and don't plan to be partying much.

 

N

Mine was epoxied from new, and after 20 years the hull was in superb condition, a day washing and a day with wire brush on a drill, then three coats of Jotun 90, all done over eight perfect summer days. Boat fairly scoots along, and very smart  I used SML for the kit, good price, good advice. 

Edited January 27, 2022 by LadyG

[blackrose]

4 hours ago, BEngo said:

Then you are pressing too hard. You really only need to let the brush skate over the surface and take the chalking off.  The brush will not clog like abrasive paper.

 

On the other hand, a wire brush will not remove all the rust.   An abrasive disc will.  

 

Why would you need to take the chalking off? It doesn't affect the performance of epoxy or interlayer bonding for new coats. Also surely a sander is much quicker than a wire wheel for keying the surface and much more uniform.

 

A wire brush on an angle grinder will remove almost all the rust if you do it properly. You can get the the bottom of pits with a wire wheel but you won't with an abrasive disc unless you take all the surrounding steel off! Anyway, on bare steel you only need to achieve surface prep equivalent to SA 2.5 for epoxy. Any decent surface tolerant epoxy will easily cope with remaining microscopic rust or new gingering.

 

On the other hand an abrasive disc will remove all the rust + good steel. You could easily take half a mm or more of good steel off without even realising.

 

I've posted this image before. It's the bottom of a locker prepped with an angle grinder & wire wheel. It's not perfect as I was leaning into the locker with all the crap hitting me in the face which wasn't very comfortable and I couldn't really see what I was doing with all the dust in such a small space. It probably needs a bit more work, however it's easily equivalent to SA 2.5 so good enough for most surface tolerant epoxies and it demonstrates that unlike ah abrasive disc, you can get to the bottom of pits with a wire wheel - without removing good steel. 

 

Anyway, fortunately everyone is free to do what they want on their own boats.

 

 

 

Edited January 27, 2022 by blackrose