New member. Need help on sanding a hull

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PEN GWYN
Posts: 2
Joined: Sat Mar 07, 2020 4:54 pm

New member. Need help on sanding a hull

#1

Post by PEN GWYN »

Hello Forum.
New member here, and I would like your experienced help.

I recently got delivery of the hull of a 43 ft sailboat. The hull was professionally built, and professionally blasted and epoxied below the waterline.
Finishing the hull above the waterline - which remains unpainted - I want to do myself (sanding, polishing (if needed?)). I want to obtain an even, clean looking surface without scratches, which should then turn to an even dull look, once on the water.
So there is no intention to get to a bright shiny, mirror-like surface.

At this stage there are still gross sanding/swirl marks (see attachments). The information I got was to use a rotary sander with 50, 80 and 120 grit sand paper.
I started on a testzone - which will later be covered - with mixed succes. The 50 grit paper (regular sand paper) seemed to leave relatively similar depth marks than how it currently looks. Moving directly to 80 grit (sianet type paper) seemed to be one step too far already, getting the surface parts even but leaving the deeper scratch marks.
We use a Festool RO 150 FEQ rotary sander.

Anyone willing to share his/her experience on how to attack such a job?

Thanks
Philippe
SV PEN GWYN
Attachments
PEN GWYN swirl marks 1.JPG
PEN GWYN swirl marks 1.JPG (215.97 KiB) Viewed 1686 times
PEN GWYN swirl marks 2.JPG
PEN GWYN swirl marks 2.JPG (195.96 KiB) Viewed 1686 times

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
Posts: 1503
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: New member. Need help on sanding a hull

#2

Post by kmorin »

Philippe, welcome to the AAB.com Forum and thanks for posting your question.

I think the idea behind the concept of beginning with 50 grit was to make the deeper scratches/sanding 'tracks' all become uniform? So that when you moved to a finer grit- the 80 grit would be refining the scratches as they'd all previously been made 50 grit in cross section or 'track' size? And it would be common in graduated grits to keep after an area until all the previous grits' tracks were gone.

While the idea that you could use rotary sanding to obtain a finish, I'm pretty sure you'll find the tool will always leave rotary 'tracks' or patterns on the surface until you get to 180 or 220 grit!! That is a lot of sanding - grading upward from 50-60 to not have scratch/tracks showing at 180 or so? If you choose to use rotary sanding, I'd suggest experimenting with a soap and water mixture as both lubricant and to keep from making as much dust- yes it a messy process but.... dust is a mess too. The sanding disks will tend to load up with sanded off metal so the soap and water mixture help there too- prolonging disk life.

Also, using 3M (tm) brand Scotchbrite (tm) pads on a rotary sander/grinder will leave fewer scratches if you begin with the brown (coarse) and graduate to the blue (fine). But there will still be a tool pattern unless you continue with even finer grits of polishing to remove the 'buffing' marks of the rotary buffing pads. I guess the random orbital mode of the sander, in the finer grits may leave very little scratching- and that would be less distinct a pattern?

I think that sand blasting would give a more uniform finish in less time- using a medium like soda or very fine silica sand should take out the previous finishing marks and leave a uniform dull grey surface? I do realize that setting up to blast and cleaning up afterwards is an expensive effort- but compared to sanding the entire topsides half dozen times? It seemed worth mentioning?

cheers,
Kevin Morin
Kenai, AK
kmorin

Chaps
Donator '09
Posts: 2173
Joined: Sun Jan 13, 2008 12:19 am
Location: Seattle, WA

Re: New member. Need help on sanding a hull

#3

Post by Chaps »

Philippe, media blasting can create a uniform appearance very quickly but most products are gritty and will leave the surface profiled (resembling sandpaper in roughness). If you can find a blasting contractor that offers a process called sponge blasting they have a media that can put a dull satin type finish on aluminum that looks consistent and allows the metal to oxidize evenly. I have a blast shop in Seattle and plan to add that process in the near future and will be offering topside restoration on bare boats that are getting rough looking.
1987 24' LaConner pilothouse workboat, 225 Suzuki
Image
please view and like: https://www.facebook.com/bottompainting/

PEN GWYN
Posts: 2
Joined: Sat Mar 07, 2020 4:54 pm

Re: New member. Need help on sanding a hull

#4

Post by PEN GWYN »

Hello.
Thank you for the feedback.

I thought about sand blasting, since the boat was going to the blasting/painting company anyway. The hull under the waterline was blasted and epoxied, before we had the hull transported to the yard closer to home, where the interior will be finished. The builder indicated that blasting will indeed leave a pitted type pattern (orange peel look, see 'attachment'), which you then need to sand again, potentially taking more material away than with if starting with sanding from the start. I guess the shop does not have soda blasting or other techniques, so that option was out.

Now, just to be clear again on the objective. I understand that scratches and swirl marks, even deeper ones, will even out over time with the boat being at sea. Some leave without doing anything (see 'hull unfinished at sea'). But that may take some time. Thus I would like to get a somewhat better look prior to launching (similar to 'KM pre-blast'). And especially also for the deck, where, apart from the look, it would be uncomfortable having such a rough surface.

The builder used 36 grit paper as his last stage. A corrective remark on my first post. I checked again, and the paper we used was 40 grit for the test, not 50. So, that was probably too close to the 36 to make a difference. Considering I continue with the rotary sander, which is pretty much the only option to proceed, what would be an appropriate next step? 60, 80, ....
And working with scotchbrite pads would possibly be an alternative starting at what stage?

Thanks
Philippe
Attachments
hull blasted.png
hull blasted.png (328.79 KiB) Viewed 1383 times
hull unfinished at sea.png
hull unfinished at sea.png (330.29 KiB) Viewed 1383 times
KM pre-blast.png
KM pre-blast.png (286.42 KiB) Viewed 1383 times

Chaps
Donator '09
Posts: 2173
Joined: Sun Jan 13, 2008 12:19 am
Location: Seattle, WA

Re: New member. Need help on sanding a hull

#5

Post by Chaps »

Where are you located?
1987 24' LaConner pilothouse workboat, 225 Suzuki
Image
please view and like: https://www.facebook.com/bottompainting/

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