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Fsatjeff
11-27-2016, 11:11 AM
Want to avoid that ugly grain rising issue. A few people recommended:

1. A thin fiberglass application, with 4 oz cloth

2. Painting the surface with epoxy first, then the color coat, and

3. Avoiding fir plywood altogether!

So, what's the opinion of y'all out there?

Thanks,

Jeff

Reynard38
11-27-2016, 11:28 AM
Gloss finish? Dark color? Short of epoxy and glass I don't think you'll be able to avoid it with DF ply.
I used DF marine ply where it would be covered with Zynole and epoxy, non skid deck surfaces.
The interior that needed a slick finish used DF MDO.

jackster
11-27-2016, 11:40 AM
Raised grain is the process of the grain absorbing the paint or whatever, then expanding, causing a rough surface texture. Many species do this to some degree.After the prime coat, the surface is then sealed and should not raise again. So sand after it drys and apply finish coats.
Fir ply is prone but also tends to check in the outer plys, so needs filling.
What parts is the ply being used for.

Spokaloo
11-27-2016, 01:14 PM
I've had great success using MDO if you want the cost and durability of fir but not the messy appearance.

The bare minimum I've had success with is 4oz cloth over fir, and it still requires a bit of touch-up to get the grain hidden.

E

Jay Greer
11-27-2016, 02:25 PM
Larry Pardy and I learned from Art Clark a trick for painting plywood and planked topsides as well. Art Was a boat builder while Larry and I were still in knee pants and he knew a thing or two about the tricks of the trade. Art and most of the painters of his age knew that white primer is a product that is used to fill the grain of wood for the purpose of getting gloss coats on a hull faster than by the correct method. Art was a firm believer in turpentine for thinning paint as, in his opinion,
it adds a bit of flex to the paint. The trick is to coat the wood with a material that will come and go as the wood expands and contracts. So many thin coats of oil based paint will soak into the grain of the placing or plywood and form a true bond with the wood. Primer is harder than enamel in its ability to flex and is intended to be easy to sand. It is not durable or weather proof and so must be top coated. Plywood is made of veneers that are unwound from the log like a window shade and, for this reason the wood has a tendency to return to its original state by microscopically flexing and causing the grain of the wood to crack under the finish, eventually forming actual checks in the wood itself. This leads to delamination of the plywood and eventual rotting of the inner cores.
This can be prevented by saturating the outer veneer layer is with the gloss enamel paint without using primer. So, using oil based gloss enamel that has been thinned with turpentine out so that it will soak into the wood is the best game in town for a lasting finish on plywood and solid planking as well. It will take a lot of time and material to accomplish the first paint job. But, after that the work is much easier. Or, you can bite the bullet and have a crew of pros come in and spray LP which can last as long as fifteen years or more. It is expensive but lasting. The savings are in not having to paint every season or two.
Jay

alkorn
11-27-2016, 04:29 PM
I have a 12 ft skiff who's bottom is made of 3/8" AB exterior yellow pine plywood. Not fir, but close. The inside was painted with an oil-based porch-and-floor enamel, first coat thinned. The outside was first given a coat of epoxy with no fabric, then painted with the same paint. 6 years old, generally dry sailed, but a week or so constantly in the water each summer. The inside has had a constant problem with the paint cracking with the wood's expansion/contraction, while the outside has been generally OK. I'd re-do the inside with epoxy but I'd have to get it down to truly bare wood first.

I don't know how well this applies to your problem, but it's a data point.

VictorBravo
11-27-2016, 05:49 PM
So, using oil based gloss enamel that has been thinned with turpentine out so that it will soak into the wood is the best game in town for a lasting finish on plywood and solid planking as well. It will take a lot of time and material to accomplish the first paint job. But, after that the work is much easier.

That's interesting. I did something like this on my scrapwood dory built in 1998 (out of cheap 3/8 doug fir plywood leftovers). I had some Rustoleum laying around. It needed thinning so I added turpentine and painted the doug fir plywood sides raw. I remember it being pretty runny, so I kept adding coats as time went by.

18 years later of abuse, neglect, sitting in the sun, and the maybe every other year splash of paint, and the sides only have minimal checking.

I added a foredeck later and tried the primer approach. After all these years, it is checking pretty badly.

I suppose it could be because the foredeck has more direct sun exposure, but I never put together the idea that painting thin oil based paint on raw was a potential solution.

BTW. I put on a new bottom this last spring with 1/2" doug fir marine plywood. I decided to try a form of boat soup on it, using raw linseed oil and turpentine--nothing else. I soaked it several times over a few weeks. It's pretty nice for a surface--no slip. But oddly, toward the bow I'm starting to see a little checking. I think that might be because the bow was a little high on the trailer and the soup all flowed aft instead of soaking in thoroughly there.