need some clarification to glass some plywood

Hi,

I know this has been answered 1 billion times or is probably common knowledge that somehow alludes me googling and searching the forums...everything i see involved recoring.

My latest fglass project is replacing a star board compartment door with a glass one. i'm just using plywood to core it, it's not a fancy boat or anything, i'm just dabbling some more. Built a form, waxed , pva'd,etc first time coring something so something simple to start there with

My questions are:

Do I pre-wet the ply with resin outside the form and let it cure before laying it or don't bother pre-wetting it and wet it out after placing it on the first couple layers of glass.

I'm assuming since it will suck a lot of moisture brush some resin on and cure then..say next day

wipe resin/ply with acetone, lay couple layers glass in form, let it get tacky a good bit, weight down the ply let things cure. Next day glass the back side. I'm assuming not to do back and front all at once.

Or do i let the first front/bottom glass layers cure,next day pour some resin and thickener and bed the ply in that, let cure, then next day glass the back side.

Thanks.

 

My understanding is that it's not important for the bonding to wood that epoxy soaks in well. It is however important to avoid epoxy starvation. You should avoid letting one layer cure before the next is applied, wet on wet is prefered. If it is practical I apply epoxy to the plywood first, then put on the glass weave and more epoxy so it is properly wet. If needed add additional layers wet on wet. Some times it isn't practical to wet the plywood first so then I simply put on the dry weave on the dry plywood and let the epoxy soak through it all. I've not noticed any difference as to the outcome.
Erik

 

Hi Erik,

I assume the same with poly resin? Also for clarification i'm going in a mold..think hatch/door type stuff, so few layers of glass, then intend to put the plywood on there, followed by layers on the back. Front will then be cleaned up and shot with gelcoat as i've done in the past with plugs i've built. I've just not encapsulated anything like plywood.

Most of what i've been able to read is re-coring, in which the front layer is obviously already dry. I think i'll just seal the ply with resin before hand, then the next day wet out and lay my first layers in the mold let that get tacky so it's still chemically active but firmer, lay and weight the ply down, then when things are still tacky remove the weights, follow up with my filler and back glass layers to encapsulate it all.

 

At the beginning of my boat-building adventures I used polyester/glass on plywood because polyester is much cheaper than epoxy. My experience from that is to never use polyester on wood again. My very first (and very uggly) boat made from polyester/ply made a fine contribution to the city dump. On an other boat I replaced a rotten deck with polyester/ply. I solved that mistake by removing the polyester with a belt sander and then covered the ply with glass/epoxy. The epoxy solution still holds.

I don't have any recommendations on how to use polyester on ply because I don't think there is any good way. I admit that my experience with both polyester and epoxy is limited, but many posts on this forum voice the same experience as I have.

Epoxy is water/oxygen tight, but polyester is not. Both water and oxygen will migrate through the polyester and the wooden core will rot. In addition to that, polyester doesn't bond well to wood.

Erik

 

I'm not exactly sure what you're attempting, maybe a better description, but polyester can work over wood. Yep, I said it, but there's a huge but. The polyester needs to be quite thick and the wood saturated with resin first. In an abusive environment (hull shell, high loaded parts, etc.), then you'll probably have to resort to mechanical fasteners to keep the fabrics in place.

As mentioned, wood and polyester don't bond well. Personally, given the volume of resin and material you need to make it work on wood, you'd save a fair bit just using epoxy. You'd use considerably less fabric, the wood bond issue goes away, it's waterproof and stronger too. To me, the savings is in the end product, once the total bill and product's attributes are added up, are the real goals. So what if the resin costs a little more, you gain more strength, stiffness, water proofness and a good wood bond, with less in 'glass fabrics. I see nothing but win, win if the project big picture is looked over.

It sounds as if you're making a 'glass covered plywood door to replace the StarBoard door. If this is the case, use epoxy. It's a lot easier to deal with and you don't have to fuss around with other stuff. Cut the plywood to shape, skin it with 'glass, then fill the weave and paint.

 

Hi , Thanks for all the great info, i've learned a bit from the discussion. That's a good point about resin, material and what you gain. You've convinced me.

Probably won't do it here as i already have the materials left over from my last project and given this is a pretty non-critical simple project. i could redo it if need be...next project though like this i'll move to epoxy.

it's really just my first dip into pulling from a mold (have made a form), but i wanted something very simple and quick to get my feet wet with wax, pva, clay fillets for corners, gelcoat in the mold first vs spray afterwards, what can i expect using mdf,etc. simple, nothing like what i see guys do on this site, a learning experience for me.

For the curious I'm basically replacing this door under the windscreen hatch (some old pics from when i did that):

https://picasaweb.google.com/104898890741999478241/FiberglassFabricationGelcoat#5728476770866205458

https://picasaweb.google.com/104898890741999478241/FiberglassFabricationGelcoat#5728476816296488050

Except it's going to be flush with the rest of the console and the windscreen hatch will sit over a lip so it's flush with that...instead of just a flat pvc slab there. Entirely cosmetic but it was just something i thought of as a first simple mold project. I'm going to use the old star board door to make some other things (cup holders,etc) when i do the upolstry.

That contoured windscreen hatch was my first fglass project, not perfect and i'd do things entirely different but that's to be expected. live and learn my goal with everything here is learn and hopefully pass the stand 5 foot away test.. Just a run of the mill deck boat i dragged out from behind someones house ...with a wth was i thinking afterwards....kids and wife like the ride on it and room...we had a pontoon boat before so it's familiar to them.

I've painted the outboard, dashboard and made a few alum panels, built the hatch, patched and touched up gel coat here and there, starting to learn a bit of upolstry. No experience with this stuff before this. On down the road with something else i'd like to be more knowledgable.

Got the mold ready, did a test with a test scrap mold today all's good ..so i made it through the what is wax, how to use clay to fillet a corner, what kind of finish to expect and how much finish sanding when using mdf...those basic type questions a newb would have.

 

Sounds like your crafty...should be fine...maybe a future in fixing old boats even and find a partner to do the engines while you do the cosmetics......then sell 'em on Craigs...lol..good luck

 

You can do what you want, but a mold is for pulling parts repeatedly from and not the best way to produce a one off product. Simply stated, with a mold you're making the door twice. Why? Of course if you need several doors, then a mold makes some sense.

 

poly seems to sit on the wood surface but I have tried with success heating the ply with a hot air gun as I work down a seam and putting the poly on whilst the wood is hot. as the air in the wood cools it pulls in the poly and the job is much stronger...

If this wood panel is to be fully encapsulated as a stiffner in the door it cannot get out or move so should be ok with poly.

 

i see what you guys are saying, i could use ply, layup, then if it doesn't bond i end up with bit of a hollow mess. ok i see epoxy or a poly friendly core would be best. pistnbroke right it's just to use as a siffener. I've learned my lesson here, especially since these are small projects i attempt the cost difference is negligible for the advanges...i was attempting something quick to save a few bucks since it's just a learning experience but i've waisted more time trying to do that =)

par the door isn't a slab it's contoured more like a hatch and a lip...bit hard to explain but just makes more sense to me to build a mold box and sculpt my lip,etc that part didn't take any time at all.

Though...are a ton of cheap production type boats like these cored with ply? don't get me wrong i know about rot,etc..i've hacked on a bunch of pieces on this boat to cut out speaker holes, etc here and there and it's mostly plywood in good shape. I'm sure in the bilge area is the usual basket case here and there.

So my curiosity has me ask:

if poly is so hard to bond to ply as it sounds here there are many areas of this boat pretty well bonded, i'm sure it's not epoxy. Don't get me wrong, i can cut a speaker hole and with a lot of work peel it off but it's stuck pretty *darn* well and i can't see that they've done anything special, do they coat the ply with epoxy or something first? when i look around...water seems more of a problem than bonding.

 

or i guess maybe it's vaccume bagged or something when they do this so the bond is a bit better?

[edit]

like there are areas, console,etc some are encapsulated, some are clearly just backed with ply then maybe some gelcoat shot over the backside of the ply..i guess
it provides stiffness in these areas. I hear what you guys are saying, i'm just wondering how a run of the mill 10 year old neglected boat is largly intact bonding wise in these
areas if poly/wood adhesion is so poor? i assume it's poly, it's just a run of the mill older bayliner

 

It's a chemical thing, not a technique thing, in regard to the bond. Polyester does stick to plywood, fairly well, but in highly loaded (unlike your door), wet locations (splash wells, transoms, bilges, etc.), enclosed places (buoyancy tanks, lockers, etc.) and highly flexed locations, the bond will be subject to enough stresses that alternatives are often sought. Also matched up to epoxy, there's just no comparison in every way you measure things, but it's your call.

Most of the major manufactures have started using different cores and resin mixtures to address these and other issues. Cost is the usual driving force behind these changes in the industry, not what works best. Volume is their business, not quality. Since your labor is free, you can cost effectively employ a much better system. Custom yacht builders, with clients that insist on the best or have cutting edge racers, typically rely on epoxy structures.

I can see your door is a seemly complex shape, though once broken down into it's separate "planes" it's just a few pieces of plywood, with some rolled shapes worked into filler or glued on foam. My point is, the mold aspect is like building the door twice, once for the mold, once for the actual casting.

You say the mold didn't take "any time". This is counter intuitive in mold making, which commonly takes a lot of time, to get the shapes right, fair and smooth the surface, then prep for the casting. Take a good look at your mold, because this is what you're going to get, dings, divots and scratches and all.

Can you post a picture or two of your mold? I'm sure a polyester over plywood door will be just fine, considering it's role and location. I just find epoxy so much easier to work with, particularly on wood, that it's not much of a choice with me.

 

Re the plywood, ahhh ok i understand! Right that's kinda what i thought, i wouldn't think of going that route for anything structural or wet locations. I was getting the drift that under no circumstances even on budget type things would that work.. thus my confusion when looking around this boat.

Actually i misworded there it's not really concave, i meant the lip, it seemed quicker to do it this way as fairing would be much quicker. It's also not square so used the existing star board door for a template and i've fillet'd the corners to match the curves of the console and entry

https://picasaweb.google.com/104898...key=Gv1sRgCPzc_sWotP6n9wE#5735923393986626274

So plan is a few layers of mat, ply for stiffener, mat, not concerned with back so that wont' be finished, just shoot some gelcoat over it.

When i made a form for the hatch there was a lot of fairing i had to do (but i've learned from that i would do it entirely different). Here i was hoping to avoid that with the front area...but i've since realized that with mdf unless i use some primer or something like laminate i'll have a bit of fairing still to do.

But i did a test piece the other day with a same thing from scraps and it seemed smooth enough to fair out pretty quickly. Since i'm going to have some fairing afterwards I see i'll shoot the gelcoat afterwards like i did with the hatch as opposed to shooting it into the mold. (i originally intended to want to try that as opposed to reading but i'll save that for something else).

 

I think i see your point though if this was something more than a flat plane you'd certainly be building it twice by going a mold route. Like the hatch i made that was all curved ,etc I didn't do a mold for that, i sculpted it then glassed over.

 

If you just glass around the edges of the ply panel overlapping about 1 inch it will be just as strong as glassing over the whole thing....maybe a 2 inch hole in the middle and glass through that producing a glass rivet holding back to front........