Top layer paint removal, how?

To answer some of you.

1/ the boat is steel
2/ no history on board regarding to the paint
3/ I have already purchased the paint
4/ I am not looking for perfection, only longevity
5/ the black primer you see is very solid and smooth
6/ at this time, I’ll only focus on the large side sections, which I cannot reach easily

Any large 9 inch sander will work, you might be able to rent one for a day.

A thick 1 1/2inch or 2 inch foam pad and a very light touch with silicone carbide (Floor sanding paper) # 12 or so glued on with disc adhesive..

This will look like cardboard with small stones glued on , but it seldom gets dull and never gets plugged up.It comes in a roll, snap off a square piece , leave the corners on .

Light touch,Light touch,Light touch,Light touch, maybe an hour a side.

You dont want to paint over smooth surface. It needs to be sanded a bit to give tooth for the paint to stick. If you prime everything you might get a smoother look and avoid print through of the black.

This may seem controversial, and the boatyard might show concern, however you might explore the containment approach and do a dustless blasting of the entire area to be painted. It'll leave bare metal for which there are excellent primers, and as long as the primer chosen is compatible with the paint you already have, you should be golden. Blasting is fast and the results produce an ideal surface for painting.
Just an alternative approach to consider...

TStiles said:

This may seem controversial, and the boatyard might show concern, however you might explore the containment approach and do a dustless blasting of the entire area to be painted. It'll leave bare metal for which there are excellent primers, and as long as the primer chosen is compatible with the paint you already have, you should be golden. Blasting is fast and the results produce an ideal surface for painting.
Just an alternative approach to consider...

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The yard allows sand blasting, but I don’t have time for it now. There are several, more important, issues I need to address, first.
The paint job on the top is mostly for aesthetic purposes. The boat looks really ugly, as it is now.
Plus, the black cover (industrial primer?) is very solid and even. I do not want to disturb it.

I think Fred may be joking, but it might work for a heavily painted bottom. Even I would be hesitant to do that though.

Attached is #12 with an #80 disk for contrast.

Rob

sean9c said:

It's a steel boat, it doesn't look like it's been faired, so it's lumpy. You're not going to be able to sand the topcoat off the low spots without, at the same time, sanding into the primer, or more likely sanding through the primer on the high spots.
Start around 80 grit, stop around 180-220 grit.
N

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This is the correct answer. (Just based on my professional opinion painting at a boatyard for five years)

Datenight said:

I think Fred may be joking, but it might work for a heavily painted bottom. Even I would be hesitant to do that though.

Attached is #12 with an #80 disk for contrast.

Rob

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Might as well use #1 grit. LOL

What's the paint that you purchased?

LeoKa said:

To answer some of you.

1/ the boat is steel
2/ no history on board regarding to the paint
3/ I have already purchased the paint
4/ I am not looking for perfection, only longevity
5/ the black primer you see is very solid and smooth
6/ at this time, I’ll only focus on the large side sections, which I cannot reach easily

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"I think Fred may be joking, but it might work for a heavily painted bottom."


No joke ,fast and sure, just be sure to use a very light touch.

If you want any hope of having good adhesion to the existing primer, you are going to have to sand it, PER the instructions on your topcoat paint. The instructions will specify what grit the underlying coat must be sanded to, usually in the 180-320 range. You can use this step to remove stubborn bits of white at the same time.

You also need to know compatibility of the paints. Since you don't know what was there, the best you are going to be able to do is determine if it was one part or two part paint that makes up the black colored layer. Take a rag wetted with some acetone and rub a small section of the black paint with it, if the paint comes off onto the rag, it's some kind of one part paint and compatibility issues are likely. If that's the case you should be using at a minimum the recommended primer prior to the top coat instead of trying to put a top coat directly on the unknown black paint.

If it doesn't come off easily with the acetone rag, it's some kind of two part paint (hopefully an epoxy primer) and your top coat is more likely to be compatible no matter if it's one part or two part, but best results will still be achieved by priming with the primer listed on your paint can's instructions.

what SBMAN says....

I'm guessing that the topsides is steel. You will be surprised how easily the white comes off with a random orbit sander and some 80 or 60 grit disks

pete

sean9c said:

What's the paint that you purchased?

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POR-15 top coat and primer.

FF said:

"I think Fred may be joking, but it might work for a heavily painted bottom."


No joke ,fast and sure, just be sure to use a very light touch.

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Yes, it looks scary on the photo, but you might have a point.
Hand sanding is not fun, but holding an orbital sander for hours, is not a joke, either.

I will give it a try. I need to practice the ‘ very light touch ‘ method. It is not something I am used to.
On the other hand, if this method helps to remove the white top layer somewhat, it could go much faster. Again, it does not need to be perfect surface.

LeoKa said:

Sounds good, but I don’t want to spend $650 bucks on a sander. Although not cheap, but I’ve ordered the Bosch GET75-6N sander. I hope it will serve me well. The price $284.00

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You made a good choice. That Bosch sander may not last as long as the Festool, but its a great one for the job and is as . It also has the dual orbital/circular mode, and is almost as aggressive as the Festoolwith a 4.5mm orbit radius.

The only issue you might have is the backing plates may be a weak point. The plastic rivets could possibly break, but easy to repair.

https://www.boschtools.com/us/en/boschtools-ocs/random-orbit-sanders-get75-6n-200696-p/

Well that's different. It's a DTM paint so I guess that you don't have to care if you sand through the primer. Trick is going to be getting all the crappy topcoat out of all the pits, welds and low spots
You think that hand sanding or using an RO sander is bad try hanging on to a big grinder like FF suggests.
Not sure why anyone would suggest that you need Festool or even Bosch other than it's not their money. Buy a 6" Porter Cable random orbit, it's like $150 and a beast. It won't do fine finish work like the other 2 but you're not doing that.
Try starting with 80 grit but have your eye on something coarser in case you need it. Use hook and loop not PSA.
Guessing you plan to roll on the POR 15, the stuff won't give you a yacht quality finish so there is no point knocking yourself out with fine grit sanding so that you don't see the sanding scratches.

LeoKa said:

POR-15 top coat and primer.

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sean9c said:

Well that's different.


Try starting with 80 grit but have your eye on something coarser in case you need it. Use hook and loop not PSA.

Guessing you plan to roll on the POR 15, the stuff won't give you a yacht quality finish so there is no point knocking yourself out with fine grit sanding so that you don't see the sanding scratches.

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I think your suggestion is very much on point. It does not have to be superb surface quality. As long it holds well, I’ll be happy.

I’ll look around for that sander.

Smooth black primer on a steel boat...

Are you sure it is not rust converter?

Northern Spy said:

Smooth black primer on a steel boat...

Are you sure it is not rust converter?

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I have no idea. I see the same on the commercial fishing boats.
It looks very solid and even. Smooth, no chipping, no peeling. Only the white on top of it looks like sh...t.

I'm guessing that it''s that Jotun epoxy primer or something like it that shipyards use. Tough as nails and you see them spraying it on in just about any weather

LeoKa said:

I have no idea. I see the same on the commercial fishing boats.
It looks very solid and even. Smooth, no chipping, no peeling. Only the white on top of it looks like sh...t.

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sean9c said:

I'm guessing that it''s that Jotun epoxy primer or something like it that shipyards use. Tough as nails and you see them spraying it on in just about any weather

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That is good news. Do you know, if they sell this to the public?

If you cant do it right at this time. Just rough it up, remove any flaking areas and painnt away. Later you can decide to blast it off and do the fine job.

You'll have to do the googling on that one

LeoKa said:

That is good news. Do you know, if they sell this to the public?

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For those that have never had the pleasure of using floor sanding paper glued on sanding disc adhesive , the grit is silicone carbide , sharper and faster than the aluminum oxide of most sandpaper.


Outlasts sandpaper 5 or 10 times longer.

FF said:

For those that have never had the pleasure of using floor sanding paper glued on sanding disc adhesive , the grit is silicone carbide , sharper and faster than the aluminum oxide of most sandpaper.


Outlasts sandpaper 5 or 10 times longer.

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I'm pretty sure that's the same stuff used on the good mesh discs as well. The difference between cheap sandpaper and spending up for the good stuff is huge.

I would say don't bother buying a sander. The yard will have a good sander for the bottom paint and you can rent it by the day. Use their sander and try the disks they have in the store. They will cost more buying them one at a time but you can see exactly what grit you etc without a big investment.

If the white doesn't come off with a pressure washer and / or a manual paint scraper then it is probably not going to come off on its own. That means you can just rough up the surface (give it 'tooth' as others have pointed out) and prime it.

This sounds like one of your first times painting / prepping. Think of it golf. This might be a par 3 project. The first shot is this time and you want to get close to the green. The second shot closer to the hole and finally knock it. That makes 3 times you might sand and paint and it might be 5 years in between efforts.

The ridgid orbital will get you a long way but no where near as fast and the big sander you can rent at the yard.

Looking at possible age of vessel and possible coating system/products I would guess the black primer is a coal tar epoxy which while a very hard and successful primer in its day presented overcoat adhesion problems if not properly undercoated and topcoated within a required curing window. I would say if the white coat present and will not come off with high pressure water blasting fair the edges hard spot prime and full coat undercoat and top coat. Not a perfect solution but give your time constraints most probably the best for any chance of success

Andiamo2018 said:

This sounds like one of your first times painting / prepping. Think of it golf. This might be a par 3 project.

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It is not the first time, but I admit, there is lot to learn about pleasure boat painting. When I was an OS on cargo ships, it was easy. They gave me the primer, later the paint, and it was never ending. Cargo ships rust a lot.

I’ve received lots of good suggestions, so I am ready for the job. I think I have everything together. All I need is dry weather. Although, the yard told me that any type of sanding requires to cover the boat completely. So, my first move will be to tarp it up.

If you have problems with your chosen top coat staying on you might try Petit Tie Coat Primer. https://www.pettitpaint.com/products/primers/below-the-waterline/tie-coat-primer/ I've had success going over old unknown coatings. Read the instructions, it's very particular for top coating and recoating. No guarantees it will work with your old primer and new top coat. Test a small area.