Best way to waterproof plywood transom

Joined
Aug 9, 2021
Messages
58
I'm remaking the transom of my 13ft alluminium boat and have 18mm marine grade plywood that I'm going to use, I'm just wondering what is the best sealer to coat the plywood in, in order to make it as waterproof as possible, thanks
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,538
The best is Woodonglass's "oldtimers formula" 1 part boiled linseed oil, 1 part spar varnish, 2 parts mineral spirits. let it soak in for a few coats, then cover with spar varnish.
 
Joined
Aug 9, 2021
Messages
58
The best is Woodonglass's "oldtimers formula" 1 part boiled linseed oil, 1 part spar varnish, 2 parts mineral spirits. let it soak in for a few coats, then cover with spar varnish.
Thank you very much for the invaluable information. Just wondering what males that formula better than if I was to just use multiple coats of spar varnish on its own? Appreciate it
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,739
Don't use wood.----Use a composite material.----Never worry about rot again.
plywood IS a composite.
It provides both strength and flexibility, along with vibration dampening.
Properly sealed/preserved, as described above, it should last half a century or more. Probably longer if it is stored dry.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,538
What did you do to the treated wood in order for it to be kept for so long without any rot?
to clarify, he is not using pressure treated.

he used the old timers formula on wood to water proof it.
 
Joined
Aug 9, 2021
Messages
58
to clarify, he is not using pressure treated.

he used the old timers formula on wood to water proof it.
so my plan is to use the old timers formula with epifans clear spar varnish on marine grade plywood. is there anything that I should change or add to this plan, thanks appreciate the replies
 

crazy charlie

Vice Admiral
Joined
May 22, 2003
Messages
5,499
so my plan is to use the old timers formula with epifans clear spar varnish on marine grade plywood. is there anything that I should change or add to this plan, thanks appreciate the replies
Marine plywood should be fine on its own for a very long time.Penetrating Waterproofing can help HOWEVER if you are coating the wood with something like varnish ,I think you are inviting trouble.Anywhere you have to make a hole like for a screw or bolt,you will potentially have a spot for moisture to intrude on the wood and the moisture will be locked it by the varnish thus promoting rot.I dont like the varnish idea.Just my 2 cents...Charlie
 

mickyryan

Rear Admiral
Joined
Apr 18, 2016
Messages
4,214
Marine plywood should be fine on its own for a very long time.Penetrating Waterproofing can help HOWEVER if you are coating the wood with something like varnish ,I think you are inviting trouble.Anywhere you have to make a hole like for a screw or bolt,you will potentially have a spot for moisture to intrude on the wood and the moisture will be locked it by the varnish thus promoting rot.I dont like the varnish idea.Just my 2 cents...Charlie
yes it should be said oversize drill holes and fill with epoxy then redrill so the holes are sealed thats the way i did mine and i know i will never have a problem anything screwed to my wood gets oversized hole and epoxy then gets redrilled for proper screw size so it don't expand it and crack it.
 
Joined
Aug 9, 2021
Messages
58
Marine plywood should be fine on its own for a very long time.Penetrating Waterproofing can help HOWEVER if you are coating the wood with something like varnish ,I think you are inviting trouble.Anywhere you have to make a hole like for a screw or bolt,you will potentially have a spot for moisture to intrude on the wood and the moisture will be locked it by the varnish thus promoting rot.I dont like the varnish idea.Just my 2 cents...Charlie
I thought that the whole point of spar varnish was to be water resistant and not fully waterproof as to allow the wood to breathe? That's my understanding as to why people use spar varnish over an epoxy coating
 

Dubed

Petty Officer 1st Class
Joined
Sep 3, 2021
Messages
373
Don't know about breathing...
It does allow the wood to expand and contract and offers uv protection
 

mickyryan

Rear Admiral
Joined
Apr 18, 2016
Messages
4,214
spar varnish has flexability that epoxy dont. expoxy is also not uv resistant..
 

TriBull

Cadet
Joined
Jul 7, 2022
Messages
17
I'm remaking the transom of my 13ft alluminium boat and have 18mm marine grade plywood that I'm going to use, I'm just wondering what is the best sealer to coat the plywood in, in order to make it as waterproof as possible, thanks
On my 1974 12’ StarCraft, I used marine plywood, sealed with marine resin and then marine paint - should last another 48 years! Lol BTW, using a jack plate makes for a great ride and freed up transom space to mount my sport ladder and boat hook!
 

Attachments

  • A8855336-9274-41A9-9C3D-09F3B58FFFCE.jpeg
    A8855336-9274-41A9-9C3D-09F3B58FFFCE.jpeg
    1.5 MB · Views: 20
  • 681F3878-2CE1-4CDE-9B4C-A4EA7322B34B.jpeg
    681F3878-2CE1-4CDE-9B4C-A4EA7322B34B.jpeg
    1.6 MB · Views: 18
  • 7CB6BC54-C9DC-409A-8DD3-9873ABE165A3.jpeg
    7CB6BC54-C9DC-409A-8DD3-9873ABE165A3.jpeg
    1,018.7 KB · Views: 18
  • FC826FB0-BF09-4761-8167-04F097CA0FBB.jpeg
    FC826FB0-BF09-4761-8167-04F097CA0FBB.jpeg
    2.3 MB · Views: 19
  • 5E92065F-7EA6-446D-AC0D-50CE0F225687.jpeg
    5E92065F-7EA6-446D-AC0D-50CE0F225687.jpeg
    1.9 MB · Views: 19

tank1949

Lieutenant Commander
Joined
Apr 4, 2013
Messages
1,892
I'm remaking the transom of my 13ft alluminium boat and have 18mm marine grade plywood that I'm going to use, I'm just wondering what is the best sealer to coat the plywood in, in order to make it as waterproof as possible, thanks
How are you fastening ply to transom? A BUNCH of 1/4" x 20s????? I have had some success on regular ply using polyester (no wax) thinned 400% with acetone. The wood sucks up the mix. But wood was NOT transom. If it were me, I used pressure treated and let it totally dry and then prim and paint and best way to fasten to boat. Rot usually occurs at drain hole.
 

jbcurt00

Moderator
Staff member
Joined
Oct 25, 2011
Messages
25,034
Unless you never ever expect the PT to get wet, never ever, best to not use PT in an aluminum boat.

Its not just about transom contact, its chemical leaching and collecting in seams to. As seen in lots of topics posted here
 
Top