Plywood sealing

Discussion in 'Materials' started by Mark9898, Jun 23, 2021.

  1. Mark9898
    Joined: Jun 2021
    Posts: 2
    Likes: 0, Points: 1
    Location: Houston

    Mark9898 New Member

    Hello, this is a non-boat question that I am researching.
    My project: a DIY truck cab-cargo box pass-through interface, 16" inches wide between cab\cargo box. The box is similar in shape to a U-Haul over-cab cargo box.
    My Design: envision 16" inches of an airplane fuselage.....1ea.-3/4" yellow pine, 7 layer plywood attached to the cab & 1ea. same wood attached to the box.
    Interior ribs are 1" x 2" pine.
    1/4" birch, 3 layer plywood wrapped on the relatively flat exterior areas with 1/8" in. bendable birch wood wrapped on the corners.
    Final Goal: appear Ford semi-gloss white, no water leaks, no ply delamination for ~5 yrs.

    Caveat: this is a prototype to test twisting and possible resulting leakage therefore I am leaning on the least expensive sealing. If I invest in top-notch sealing now only to find my design is poor I have wasted time\$$$.
    My concern is the cab will twist independently of the box even though both are affixed to the same chassis continuous steel channel beams.
    After testing I can always return to a better seal.
    I have already applied several coats of water-based Latex primer paint to the 3/4" in. ply & ribs.
    The exterior skin plywood has yet to be attached or coated.
    My thinking is: paint with a water-based, white Latex topcoat only.
    If my design is good I can always return to apply: resin, urethane, fiberglass, rubber sealant etc. etc. whatever is best & recommended. I think boat people will know what is best at that time.
    btw....I am poor.
    Suggestions \ comments?
    I don't want to apply something I have to sand off later, if possible.
    My testing will be max. 1 yr.
    Eventual real life, off-road usage not for several years.
    Thank's in advance.
    Regards, Mark
    PS: project in Houston thus 73-95F with ~65% humidity.
  2. Blueknarr
    Joined: Aug 2017
    Posts: 1,432
    Likes: 402, Points: 83
    Location: Colorado

    Blueknarr Senior Member


    Welcome to the group.

    I hate to be the berier of bad news.

    You have already made several poorly informed choices that will severely shorten the project's lifespan.

    I have never seen birth plywood made with exterior grade glue. It will delaminate along the factory gluing regardless of subsequent layering or sealing.

    Epoxy is the best sealer. Period end of story. All other sealers are a distant third.

    I suggest that you finish this one with water based products.
    Put it into service and monitor how it performs.
    Then replace it in two or three years when it starts to delaminate.

    Build the replacement with exterior or marine grade plywood.

    Good luck
    fallguy likes this.
  3. Mark9898
    Joined: Jun 2021
    Posts: 2
    Likes: 0, Points: 1
    Location: Houston

    Mark9898 New Member

    Understood and thank's for your input.
    If I now replace the 1/4" in. birch ply with 1/4" in. marine-grade ply what are your thoughts?
    btw: the cab end face of 3/4" ply has a 2"in exterior caulked rubber seal so this ply should not see any significant water.
    I could replace both 3/4"in ply end caps but am resisting due to cost.
    Q1: Assuming I want to go forward with epoxy resin is the already applied latex, water-based primer on the 3/4"in. ply an issue?
    Q2: If no to Q1, the next step is to apply the epoxy?
    Several coats (2-3), 1st lightest, lightly sand with #180 between coats for adhesion, correct?
    Q3: Last step top coat white, semi-gloss with what (gelcoat?) type of paint ?
    Lastly, as I stated my design may be faulty for stress fractures so I need to keep this in perspective and....I am currently poor which will change in several months.
    Again thanks, not arguing with you just seeking knowledge.
    Regards, Mark
  4. Blueknarr
    Joined: Aug 2017
    Posts: 1,432
    Likes: 402, Points: 83
    Location: Colorado

    Blueknarr Senior Member

    Epoxy over paint is usually OK. Rarely will Epoxy cause paint to bubble up. The paint to wood bond is weaker than an Epoxy to wood bond.

    Three coats of Epoxy is recommended. Three extra coats on the edges (end grain) of the plywood or solid stock. Extra coats under the rubber weather strip as it may chaff.

    Wash epoxy with soapy water before sending to remove any amines.

    UV will quickly destroy espoused epoxy. Almost any paint will work as a top coat. If you can spray it, get color matched urethane from an auto body supplier.

  5. fallguy
    Joined: Dec 2016
    Posts: 7,398
    Likes: 1,586, Points: 123, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    Basically, bk has said it for me.

    I will add that exposed endgrains do not play well

    Any endgrain not glassed, as in fiberglass tends to open over time.

    The best solution for your box would have been plywood, 4 oz fiberglass and epoxy, then primer, then topcoat.

    I belong to a plywood boat builders group and every guy who doesn't glass the transom top ends up doing it later. The expansion rates of the glues and wood vary. Fiberglass makes the expansion rates more uniform.
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