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Old 08-07-2019, 11:00   #1
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Painting a concave surface, HELP!?

The stern on my Columbia 10.7 angles down and in at about 15 degrees. Every time I try to paint this surface with rollers or brushes the paint sags instead of laying out properly like it does on vertical or convex surfaces.



I'm using really good paint, but can't get a good result with brushes and rollers.

Anyone know the secret? Do I need to thin the paint? Use special brushes?

Last time I gave up on rollers and brushes and used canned spray paint because I was in a hurry after the first attempt failed.

Even the spray paint sagged. I had to wet sand for a couple hours to get it looking barely acceptable.
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Old 08-07-2019, 11:10   #2
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Re: Painting a concave surface, HELP!?

Is it the type of paint you can put on real thin coats and multiple? That's tough, surprised it isn't just as bad on a vertical surface.
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Old 08-07-2019, 11:19   #3
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Re: Painting a concave surface, HELP!?

Last year I used a two-part polyurethane ($130). Last weekend I tried a one-part polyurethane ($54). Not cheap.
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Old 08-07-2019, 14:16   #4
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Re: Painting a concave surface, HELP!?

It's your technique. You need to go with less paint. The first coat does not even come close to covering. After it tacks up another thin coat. That might start to cover things up. It takes 3-4 coats.

I've sprayed a number of airplanes and they have some really hard surfaces to get flat. You need good paint and thin coats.
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Old 09-07-2019, 14:27   #5
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Re: Painting a concave surface, HELP!?

Thin the paint per manufacturer instructions...using a brushing/rolling additive.... Multiple thin coats work much better than 1 or 2 heavy coats. I agree with Ecos...it may take 4 coats or more to get it right. I too used to paint airplanes in my younger wilder days.. you dont know the meaning of the words pain in the neck, till youve sprayed the underside of a wing.
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Old 09-07-2019, 17:26   #6
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Re: Painting a concave surface, HELP!?

On a concave surface a narrow roller will give much better results because the full length of the roller is in contact with the surface being painted and leaves a more uniform thickness of paint without the tracks from the edges of the roller. I've had good results with the two inch width rollers, It takes longer but gives you better thickness control
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Old 09-07-2019, 18:11   #7
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Re: Painting a concave surface, HELP!?

I agree with the other posters that your paint is probably too thick. But too thin is problematic also. It takes experience to do this. I have been painting things for so long that I can tell when the paint drips off of the stir stick if it is either way. You might try a test piece of plywood set up vertically to test the paints viscosity. Or get an experienced person to help you. You shouldn’t really need to thin the paint much to work, as they said multiple light coats is the answer.
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Old 15-07-2019, 11:35   #8
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Re: Painting a concave surface, HELP!?

I gave up trying to use a brush. I switched to a roller with a thin nap. The first coat was much better than any of the brush coats.

A long time ago, I used a two-part epoxy paint and sprayed it. The result was very good, almost professional. Unfortunately, I can't spray in the boatyard where I'm situated without setting up a spray booth over my stern
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Old 15-07-2019, 11:40   #9
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Re: Painting a concave surface, HELP!?

I can't use a brush either. I found the good 4" foam rollers work pretty well without using a brush. You need to use the slowest retarder you can get. The Algrip stuff will work in other poly paints.
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Old 15-07-2019, 12:23   #10
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Re: Painting a concave surface, HELP!?

Quote:
Originally Posted by Bigjim View Post
I gave up trying to use a brush. I switched to a roller with a thin nap. The first coat was much better than any of the brush coats.

A long time ago, I used a two-part epoxy paint and sprayed it. The result was very good, almost professional. Unfortunately, I can't spray in the boatyard where I'm situated without setting up a spray booth over my stern
You need to use both, a roller and a brush....roll it on then tip it off with a quality brush...if you roll vertically, then tip it off horizontally... or vice versa.

It wont run or sag. Theres plenty of videos on YouTube to show you how.
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Old 22-07-2019, 10:46   #11
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Re: Painting a concave surface, HELP!?

Quote:
Originally Posted by dennismenace111 View Post
You need to use both, a roller and a brush....roll it on then tip it off with a quality brush...if you roll vertically, then tip it off horizontally... or vice versa.

It wont run or sag. Theres plenty of videos on YouTube to show you how.
I gave up on brushes. I finally found the solution:

Foam rollers with thinned paint!

The regular rollers left fuzz behind no matter how many times I cleaned them. And, brushes always left brush strokes. I've tried a half dozen different brushes with no luck.

Here is the final result with foam rollers:



I pulled off the tape and cleaned up my edges with 800 grit sandpaper.



Today, I'm going after the black with 1000 and 2000 grit wet sandpaper. I'll post pictures when I finish.
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Old 05-08-2019, 16:26   #12
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Re: Painting a concave surface, HELP!?

I've finally hit the secret. Here's what the stern looks like now:



The next step is buffing out the uneven surface and then polishing it to glass smoothness.
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Old 05-08-2019, 16:44   #13
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Re: Painting a concave surface, HELP!?

Quote:
Originally Posted by Bigjim View Post
I've finally hit the secret. Here's what the stern looks like now:
The next step is buffing out the uneven surface and then polishing it to glass smoothness.


LPU is not meant to be buffed. You can buff it, but you will ruin the natural surface.
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Old 30-08-2019, 10:12   #14
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Re: Painting a concave surface, HELP!?

Sheesh. It's always something...
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