Does anyone know why fibreglass resin will not go off over the knots in plywood? I've tried it with and without knotting compound but with the same result - the area of the knot remains unhardened. I've tried heavier doses of hardener as well but no luck. There is nothing about it on Google. The job is using plywood to strengthen and floor a hotrod GRP bodyshell.
Any ideas?
Slatts
Don't use plywood would be the obvious answer. Can't you use laminate of some kind? - or wire mesh?
Give these guys a ring and ask 'em. They'll know if anyone does.
I've never found that resin fail to go off.
I conjecture its actually gone off, but something else is underneath that is stopping it adhereing.
It would help to know what sort of resin you are using. I'm guessing that it's polyester resin, i.e. the cheap stuff.
If you are using polyester resin then you're using the wrong sort of resin. It doesn't penetrate plywood nor does it bond to it particularly well. You should use epoxy resin for this. It's not cheap.
The best epoxy resin for the job that you are trying to do is a slow cure, low viscosity expoy resin. There are several available. The slow cure and low viscosity will ensure that the resin will penetrate the wood before it cures. This ensures that the resin keys well with the wood and that it wil will protect it properly. Polyester resin has a higher viscosity and it does not penetrate well into wood, hence it flakes off the surface relatively quickly - in my experience in demanding applications such as the floor of a car it won't last much longer than a couple of years before it cracks and starts to flake.
One thing I suspect that you are missing is that you must degrease the wood before applying the coating. This is best done my washing the surface of the wood with white spirit and then with acetone, as soon as the acetone has dried, coat the wood.
I suggest you try SIP low viscosity slow cure resin, System Three Clearcoat, or MAS slow cure (all expoy resins).
There are many different polyesters, not all of them cheap.
All of them are cheap compared to epoxy resins. Thanks for snipping so much from your reply that the context was removed.
... and if the ply has knots in, it's probably the wrong sort of ply. Strength-weight ratio varies enormously with different sorts. The better stuff is lighter, stronger, and knot-free.
Also rather expensive!
You might want to consider GRP skins over a balsa core. Old boat technology for strength and lightness.
Andy
I've got the suspicion that the "knots" are in fact filled with something. On cheap, knotty, veneers the knots fall out and at the factory someone used to be paid to glop filler into the holes. The filler is similar to plastic wood and I suspect that if this has been used the styrene in the polyester resin will cause the filler to soften. It may well inhibit the peroxide hardener as well.
I've had good results using WBP as the structural material, flow coating it with slow-cure epoxy then varnishing it with two-pack "crystal" polyurethane. It looks stunning when finished.
In message , Andy Champ writes
Steve Firth's reply earlier was also spot on. You are essentially looking for boat-building technology.
The only way to buy decent ply is to get into all the wood yards you can find and then inspect all the sheets they can offer. You probably need marine ply or at the very least wbp, and you need to inspect to make sure there are no gaps between the individual sheets in the cores. Any edges need to be coated and made good before gluing to anything else.
Learn about epoxy and how to use it as a glue as well as a coating. I'd want any sort of vehicle to be structurally sound, so the resin must integrate properly with properly prepared material.
Yes - It's polyester. I've used it because in the past I've made agricultral machines that took a lot of stick in all weathers and not had any problems with the GRP. But I've never used wood for strengthening before as I've made up steel frames and it was those that failed but almost always from mishandling.
I'll keep an eye on that as the body is well supported and I can easly lift it off to re-do it if nessesary.
Yes I did that.
Thanks for the info. I'll try the epoxy as I have some but didn't want to "waste it" on the floor!
Slatts
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.