glass over plywood

What is the best way to prepare the plywood, as in a floor, prior to laying glass? any thoughts would be great, thx

 

Rough up the surface with 100 or lower grit. CPES (or other penetrating epoxy) all edges, holes, cutouts, fastener holes, etc., in the plywood. Apply this until the plywood will suck up no more. Lightly scuff the surface with 100 grit again, then layup your 'glass, using one of the several different methods. This is the only way (encapsulation) epoxy can keep out the moisture, which will swell the wood and harbor rot. It must be embalmed or it doesn't work.

 

There is a way without allowing the first coat to set. lay your glass out, fold it back roll epoxy to the surface and lay glass back over, then apply your epoxy, as soon as the first coat is sticky ( when you put your finger in it it does not come off on your finger bt still sticky) apply another coat. continue untill your day is done. Don't forget to smooth glass with squeeegeee.
To get an even coat of epoxy roll with one roller (wet) and then use a dry roller to absorb excess. In most cases it's cheaper to fair epoxy then it is to fair primer.

 

duratec sells a vinyalester sealer which will seal the wood good u can also use polyester and mix 50% syrene in to it add some colbalt or dma to re promote the mix , this will be soaked up good by the plywood ,u must impregnate that wood all the resin it can take ,what ur trying to do is make that plywood into plastic and trust me plywood rots fast ,any nail or screws should have plenty of silaflex or silacone as well.I would laminate the glass first with chop mat it sticks good to wood and depending on what u want to spend a epoxy resin for the layup.also it will bond to what ever resin the boat is made from

 

Goodwrch asked "What is the best way to prepare the plywood, . . ."

Duratec's sealer isn't as good as penetrating epoxy for sealing wood. Thinned polyester is less effective then the vinylester previously mentioned, regardless of the additives or reinforcements used.

Plywood doesn't rot any faster then solid lumber, which is particularly true if well maintained coatings are kept intact.

Silicone has no business being near any fastener hole on a boat, save it for your home bath tub or kitchen sink caulk and use appropriate bedding compounds, sealants and adhesives designed for the tasks being asked of the piece.

 

Another suggestion to add to PARs good advice..I agree with him about the silicon.
I'd use epoxy...the adhesion to plywood is much better than polyester resin. Most don't use CSM with epoxy but it works fine if you take the time to make sure it is wet out.
I roll a coat of epoxy on the ply, let it soak in and gel, then roll on another coat (fresh batch) and lay on my cloth and roll it so that the epoxy comes up through the fabric. I add more epoxy from the top as needed and thoroughly roll it so that you can see no "dry" patches or bubbles.
Even though the ply should be sealed with the first coat...bubbles will appear as the layup heats the ply...the air expands,...out they come.
If you do your glassing in the evening while the air is cooling down you can minimise the problem of out-gassing from the plywood, If you start work in the morning or put heat lamps on it the opposite is true.
As for preparing the ply....just make sure it is clean and dry.
Regards
Rob.

 

I have been experimenting with pressure treated ply.
Just make sure it is well dried out. I have had great success with it and polyester resin sticks great. lightly scuff with 40 grit paper then apply the glass. even if water gets in it will not rot as with reg ply. the preserv. will not allow fungis to grow which is the reason wood rots.

 

I've had very good results using a "biscuit" scraper to apply the epoxy to the wood, before glassing. It is amazing how far a little epoxy goes. In fact, my mentor epoxies all his ply (okume) on arrival to the shop, front and back. This protects it from shop rash, as well as makes it ready to be glassed over when the time comes. Just a simple light sanding before you glass.

 

I've been around long enough to see what happens to polyester on wood, be it engineered or solid. There were thousands of catamarans and small boats built during the home building craze in the 60's and 70's, most every one covered with polyester. They can be found (those that haven't been dragged to the land fill) with sheets of cloth hanging limp on their flanks.

It just doesn't get the job done and you're not saving that much money, certainly no more effort. Those of you that are recommending poly, try to get an insurance company to sign off on a repair using this technique and see what happens . . . you'll eat a lot of work, trust me. They know what works, from having to pay out on many years of previous attempts using this material. It's not guess work, it's long proven fact.

Out gassing can be handled with a sealing coat first, preferably with a penetrating epoxy (several coats, until the wood will take no more) It can also be handled by working with a warm piece, which has been moved to a much cooler area after the epoxy/cloth has been applied.

Saving time on the layup with multiple coats and layers is a common technique, but difficult for the beginner to handle, without much 'glassing experience. Working with hardener speeds and "catching" the previous coat for the next batch to chemically "key" requires some discipline and practice, but not very hard to do.

I hope the original poster doesn't get lured into the false impression they can save some money with poly, it just doesn't work for very long and no professional restorer or repair shop worth their salt would recommend this avenue of pursuit to reach their goal.

 

what i mean by silacone or silaflex is that if u screw in to the wood cover the threads of thr screw with the sealant i never said to use it as a bedding meadium

 

Don't add large amounts of styrene to polyester resin, even small amounts are not recommended. Although styrene will mix in and get hard, it's very weak and will only degrade the resin. 5% maybe, 50% never.

 

Silicone will not seal the threads of a wood screw. It will merely act as a lubricant when driving the screw home, little else. Some products from Silaflex can be quite useful around the building project, but again it will not do much if applied to the threads of a wood screw, again getting wiped off as the screw cuts it's way into the substrate.

 

Another prep shortcut to take potshots at

Don Casey (do I have the right first name?) suggests bleaching the surface of some plywoods to remove the softer fibers and allow a better key than sanding would. The idea is intriguing and I have tried it with great success on some non-critical layups for an experiment. I know that the initial results are excellent, but I don't know about the long-term durability.

Any thoughts on that idea?

 

Bleach would not remove the fibers (scrubbing, abating, sanding will), but it will break down the sugars, that bind the dense and less dense areas of the structure together (winter and summer growth), the softer areas would be more effected, but both types of material would be.

As an adhesive, penetration can be increased with several techniques, none requiring the substrate to be physically weakened in the process. Mechanical "keying" is easily achieved with roughing up the surface (the more the better in many cases), or artificially drying the surface with alcohol so the epoxy can leach into it better. This is necessary on woods that are particularly oily in nature. Temperature can dramatically increase penetration, but it's moot in most boat building applications. The grip on the substrate will exceed the ability of the wood fibers to resist tear out, so further penetration isn't necessary.

The same is true if epoxy is used as a sealant. Tests, by several of the major manufactures, have proven penetration depth has little effect on the ability of wood to be stabilized from moisture content gains. Encapsulation is entirely dependant on the coating being intact, not the amount of penetration into the substrate.

 

Thanks!

This explanation definitely beats waiting years to see if the eperimental pieces hold up and still not knowing if I had accomplished anything useful.