Hull preparation

[unclejack3]

Hi.
Need to repaint hull so am sanding it down with 40 grit,,quite hard to get paint off, is the black paint anti foul as it was moored,and does it need to be taken off. Am I using the right sanding discs?.

 

[Davidasailor26]

It looks like the black probably is antifouling. It’s not uncommon to put a “tracer” layer of paint below the final coat, so you can see when the top coat is wearing down and needs repainting. My guess is you’re finding blue antifouling over a black antifouling tracer coat, over white gelcoat.

40 grit is pretty aggressive so you might be cutting into the gelcoat a bit. I personally wouldn’t say not to use it though - I don’t like sanding any longer than needed either. It might be best to apply an epoxy barrier coat after sanding to build back up any lost gelcoat thickness, ensure you have a good smooth surface, and prevent any osmotic blistering in the future. The discs you showed should be fine, though I’ve had and easier time with mesh disks like Abranet. I find those don’t get clogged with paint dust so much and they keep working more consistently for longer.

 

[colemj]

40 grit is correct. For this job a rotary sander is going to work better than a RO. Gelcoat on the bottom serves no purpose, and is only there because it is easier to spray the entire mold when building. Don't worry if you sand into it. An epoxy barrier coat could help stop osmosis, but if a 70's era boat hasn't suffered osmosis yet, or you trailer it, it is unlikely to have osmosis now. Sanding into the gelcoat won't increase that possibility. If you do decide to epoxy coat, you will need a clean surface free of any paint at all. This will make the job more involved.

Why do you want all the paint off? Most people just sand enough to get to good adhered previous paint and tooth it, then paint over it.

Mark

 

[dlochner]

Paint companies usually recommend 80 grit. Lately I've been using mesh sand paper, it seems to last longer, clogs less, and cuts faster. There is also less dust in the air when using a vacuum attached to the sander. A rotary disc sander does work faster than a random orbit, however it is a bit harder to control which can lead to gouging.

 

[colemj]

80 grit is the recommendation for tooth preparation of an old surface. To remove paint altogether will require coarser grit than 80. We are having this done right now (decided long ago that this job was well-worth paying for).

Mesh paper is better, particularly when toothing, but removing this much paint won't matter because one will be changing either type too much to matter. The guys doing our boat use 48" sheets of sticky back sandpaper and just cut out a large batch of discs from it each morning. This paper is sold on big rolls in all the hardware stores here, so suspect it is a common material for floor sanders or something. They go through a lot of discs, so buying bulk paper could save some money.

While perhaps a bit harder to control, a rotary sander is much faster removal than an RO. One of the guys removing paint is using a rotary, and the other a RO. The rotary guy is lapping the RO guy, and both are working equally hard.

Gouging the bottom a bit is inconsequential, as it will just be gelcoat. Any gouge into the gelcoat is easy to feather or fill before painting.

Mark

 

[unclejack3]

Hi.
Many thanks for the valuable advice.
I am using 40 grit because there are many layers of paint especially below waterline and I want it to be really smooth, above the WL I can use 80 as it's not that bad.
I see most YouTube using orbital so I assumed that was the go but I must admit it's pretty slow and hard especially lying on your back, will look into these mesh disc today.
Now for paint, since it's a TS it does not need antifoul so I should be able to paint the whole hull , so should I still use an epoxy barrier paint under the WL as a hard finish to fill in all those little dents in the gel coat then paint over it with a polyurethane primer and topcoat?
I also have a keel repair issue,rusted pivot hole, so if you have any knowledge on this then let me know as well. Again thx for the help.

 

[dlochner]

An epoxy primer will fill much of the evidence of sanding. If the bottom isn't perfect that's probably OK unless you intend to race the boat. Check the paint instructions for the top side. 80 grit may be too aggressive. If you have never used polyurethane paint you're in for a treat, it is thinner than water and will take at least 3 coats. Check the manufacturer's recommendation for grit.

 

[unclejack3]

Hi.
So should the epoxy primer be put on all the hull or simply under the WL...the above WL is fine but if the epoxy is going to protect the whole hull then why not, I assume I still need primer then top coat on ALL the hull...

 

[dlochner]

Hi.
So should the epoxy primer be put on all the hull or simply under the WL...the above WL is fine but if the epoxy is going to protect the whole hull then why not, I assume I still need primer then top coat on ALL the hull...

Your best bet for the correct answer to these questions is to identify the brand of paint you will be using and review the application instructions for that paint. In general you will want to have a good primer and a very smooth surface for polyurethane paint. Because it is so thin and shiny it will show any flaws in the surface preparation.

 

[Alan K]

Years ago, 15?, I stripped my bottom paint and applied Interlux barrier coat. One of the best decisions I ever made! I hired a sandblasting contractor, experienced with boat bottoms, through my yard to remove the old paint. There are abrasives, other than sand, which can be used and are gentler than conventional sand blasting media. The benefit of using a contractor was that he draped the boat and contained all the debris/dust. This was important since my vessel was stored among other vessels.

Once the old paint was removed, priming and preparation were easy. I put on the recommended number of barrier paint coats recommended by the paint manufacturer. Alternating colors (Grey and White) between coats made it easy to see where I had painted. I finished with 2 coats of hard ablative bottom paint (Pettit Vivid). Now I only touch up my bottom paint annually where I to see grey barrier coat beginning to show through. No excess paint buildup. Worth every penny if you plan on keeping your boat for a while.

Alan
H380

 

[ds34mail]

I used a carbide scraper to get my 20+ years of bottom pain layers off. No way I could have ever finished the bottom just using a sander.

 

[dLj]

Years ago, 15?, I stripped my bottom paint and applied Interlux barrier coat. One of the best decisions I ever made! I hired a sandblasting contractor, experienced with boat bottoms, through my yard to remove the old paint. There are abrasives, other than sand, which can be used and are gentler than conventional sand blasting media. The benefit of using a contractor was that he draped the boat and contained all the debris/dust. This was important since my vessel was stored among other vessels.

Once the old paint was removed, priming and preparation were easy. I put on the recommended number of barrier paint coats recommended by the paint manufacturer. Alternating colors (Grey and White) between coats made it easy to see where I had painted. I finished with 2 coats of hard ablative bottom paint (Pettit Vivid). Now I only touch up my bottom paint annually where I to see grey barrier coat beginning to show through. No excess paint buildup. Worth every penny if you plan on keeping your boat for a while.

Alan
H380

I did the same about 3 or so years ago. I had the hull soda blasted and then did the barrier coats. I've sanded far too many hulls in my life time to want to do it again...

A good barrier coat is very high on my list if you are keeping the boat for a long time.

dj

 

[Barnacle Bill]

My recommendation is to sand the hull so that it looks like your photo on the left half of the first photo. It looks like there is a cocktail of bottom paints applied through the years. Best to remove it all and start over. Once completely clean, now is a good time to apply a water barrier paint like Interlux' 2000e or similar according to the instructions. Then apply the antifouling paint of your choice. The slot suggests you have a swing keel boat and so an ablative, multi season paint would be my recommendation. That way you won't get a build up because the paint and toxicants shed away from the hull at the same rate and when down to seeing the epoxy (or gel coat, if you don't apply an epoxy water barrier) you know it is time to paint.