epoxy coating

[metin_mehel]

Hi guys,
I made a boat with okume plywood. The surface up to waterline is coated by fiberglass-epoxy. Remaining surface of the boat was coated only by epoxy.
I will paint the hull with Acrylic primer and paint. Should I sand the hull before applying primer? If so,do yo think the area which is not coated by fiberglass will be sanded as well? I think after sanding, the wood surface will appear.
Thanks A lot

[michael pierzga]

Yup, you must sand all surfaces to develop a mechanical bond for the paint system.

Normally , after fairing the entire hull , an addition one or two coats of epoxy is applied. This final barrier coat of epoxy is then lightly sanded and painted. Dont sand thru the epoxy coating. If you cut thru the epoxy coating , you have compromised the moisture barrier and must apply another coating of epoxy.

[metin_mehel]

Have you ever coat the wood surface with only epoxy?

[Sand crab]

Epoxy has very little UV resistance. If your boat is going to be in and out of the sun like a canoe then it would be OK. For a larger boat permanately in the water the epoxy must be coated over with something. Awlgrip is popular. BOB

[LP]

Epoxy only (no glass) is fine. You still need to UV protect the epoxy by either paint of varnish. Glass generally provides for abrasion resistance only in most light craft though it can also contibute structurally in ultralight constructions.

[metin_mehel]

I am building a catamaran. I only coated underwater surfaces. The remaining surfaces are only coated by epoxy once. I think I should coat at least twice. So first I will grind as far as I can see the wood , Then I will coat with epoxy again. But I am confusing little bit. Because coating epoxy layer is so thin and before painting the surface must be grinded. So there will be no epoxy coating on the wood surface. What do you think?

[michael pierzga]

Moisture barrier effectiveness.

http://www.westsystem.com/ss/assets/...eExclusion.pdf

[Sand crab]

One of the reasons for sanding between coats of epoxy is to get rid of the amine blush if your epoxy is susceptible to this. Check with the manufacturer and follow instructions exactly because amine is waxy like and your next coats will not stick and it will be a disaster. Do not grind off your epoxy down to wood. You only need to top coat it with the appropriate paint. One thin coat of epoxy may not give you the water proofing you need. You may need more coats. Sometimes you will need to lightly sand it to get your next top coat material to "bite" into the shiny epoxy surface. This is very common with interior finishes in construction. Once again, if your boat is going to spend alot of time covered or out of the sun then the epoxy will be OK by itself. BOB

[Herman]

Procedure for coating wood:

-apply epoxy
-let cure
-sand smooth with fine paper (P150, P180). Do not sand though the coating, just top it off to remove the swollen fibers
-apply epoxy
-let cure untill sticky to just hard
-apply epoxy
-let cure
-sand smooth (P180)
-apply epoxy
-sand smooth (P180, perhaps P220 or P240)
-apply paint following the instructions of the paint manufacturer.

This is the procedure we followed at the West System distributor in NL, and this always gave good results.

[tunnels]

Originally Posted by michael pierzga

Moisture barrier effectiveness.

http://www.westsystem.com/ss/assets/...eExclusion.pdf

Interesting reading !!

[lewisboats]

For best results and a decent thickness barrier without too much sanding, wash what you already have with ammonia and water to remove any amine blush that is there. Sand lightly with 120 to 180 grit sand paper to prep the surface and then put on a good coat of epoxy. Wait until the epoxy is firm but still a bit sticky and apply another coat. This will let you skip sanding between coats. Add another coat when this gets sticky or just let it cure hard (give it a few days) and then you can wash again and give it a final light sanding and then start painting.

[metin_mehel]

Thank you very much for all these god informations. One another question:
Think that you build a sailing yatch 12m. Should we coat the exteriour of the hull with frp fabric and epoxy? Or just epoxy can be enough?

[michael pierzga]

Depends. Fiberglass cloth set in epoxy might be a structural component.

If your design does not call for fabric , then obviously you dont need it.

The great benefit of cloth set in epoxy is chafe resistance and a moisture barrier.

When you bond light 6oz fabric over plywood , you automatically achieve the necessary film thickness of epoxy to form a moisture barrier when you fill the weave of the cloth fabric .

As you as presently finding out its very easy to SAND THRU an epoxy only finish on wood.

Cloth set in epoxy avoids this moisture barrier compromise by giving you a visual key when sanding and fairing.

[Sand crab]

12m=fabric and epoxy