Strangely, the tighter bond in my very unscientific test broke much more easily than the one involving a gap bridged by Extramite mixed with micro fibres. Probably not very reliable though - especially as the Extramite may have been past its shelf life - but I was surprised nonetheless.
One disadvantage of Extramite used over a gap is that anything more than a thin layer takes much longer to cure. It'll still be rubbery for a day or two. This compares with a fillet of epoxy where the stuff seems to harden as quickly as a thin layer in a tight joint. The other thing about Extramite is that it shrinks as it goes off (because of the water evaporating?) whereas two part glues like Epoxy don't. Means that if you have the glue line nice and flush with the surface there'll be a little hollow there the next day when using powdered urea formaldehyde.
But maybe the shrinkage has benefits too - would it pull the mating surfaces closer together to create a stronger bond?
I spoke to the technical chap at Polyvine - the company that's taken over Humbrol/Humbrol's wood glue operation - and he thought that the idea of mixing an inert filler like micro fibres into what's now Polymite was an interesting one. He said it had never been tested at their end but might be worth doing. One tip he did give me for using powdered urea formaldehyde was to use water that had been kept cool in the fridge when mixing. This replicates a cold day and gives you a much longer open time. Apparently the open time with Polymite can vary from as much as 1 1/2 hrs to only 30 mins depending on the temperature.
Another thing he mentioned is that urea formaldehydes hover just at the bottom of the b4 category in the standards for how waterproof an adhesive is. This means that while joints using UF can take a soaking they shouldn't be continually submerged in water or the resin will eventually break down. Boat builders take note - alright for dinghy construction but not the stuff to use when gluing up your rudder for that year long Mediterranean cruise in the newly built 50ft wooden yacht. :shock:
One disadvantage of Extramite used over a gap is that anything more than a thin layer takes much longer to cure. It'll still be rubbery for a day or two. This compares with a fillet of epoxy where the stuff seems to harden as quickly as a thin layer in a tight joint. The other thing about Extramite is that it shrinks as it goes off (because of the water evaporating?) whereas two part glues like Epoxy don't. Means that if you have the glue line nice and flush with the surface there'll be a little hollow there the next day when using powdered urea formaldehyde.
But maybe the shrinkage has benefits too - would it pull the mating surfaces closer together to create a stronger bond?
I spoke to the technical chap at Polyvine - the company that's taken over Humbrol/Humbrol's wood glue operation - and he thought that the idea of mixing an inert filler like micro fibres into what's now Polymite was an interesting one. He said it had never been tested at their end but might be worth doing. One tip he did give me for using powdered urea formaldehyde was to use water that had been kept cool in the fridge when mixing. This replicates a cold day and gives you a much longer open time. Apparently the open time with Polymite can vary from as much as 1 1/2 hrs to only 30 mins depending on the temperature.
Another thing he mentioned is that urea formaldehydes hover just at the bottom of the b4 category in the standards for how waterproof an adhesive is. This means that while joints using UF can take a soaking they shouldn't be continually submerged in water or the resin will eventually break down. Boat builders take note - alright for dinghy construction but not the stuff to use when gluing up your rudder for that year long Mediterranean cruise in the newly built 50ft wooden yacht. :shock:
