Cracking bog

Hi All
I've recently completed construction of a GRP trimaran so it may be a bit late to ask this question.
Throughout the build, I had problems with cracking bog. I'm using unwaxed polyester resin, Polyflex Isophthalic from Nuplex Composites. The filler is Cabosil, which is a filler suitable for epoxy or polyester as an adhesive. I use microballoons or a combination for finishing.
I've had problems with fast and slow mixes, on hot days and cold. Some of the slower mixes (24 hours plus) have cracked while still soft. I can understand a hot mix in a big volume cracking but that, generally, has not been the situation - mostly, I'm bogging a radius before laying glass to bond in a bulkhead etc.
I've worked around the problem but, on launching, I've had some leaking around the centreboard case caused by hairline cracking in the bog.
I've been using this stuff for years and have never had a problem with polyester or epoxy. What have I got wrong?
Thanks for any advice
Harry

 

harrygee,

Just how thick has the radius been that you refer to, normal (say 6mm to 10 mm) I have never had it crack, but whilst working in China, I did see very large patches of bog crack, I believe because it was too thick (in section), and generated too much heat whilst curing. Certainly in practice, when I made then do smaller sections and more of them, we had no more cracks.

 

All you doing is not rocket science !! just need to do a little thinking !!!

Bog !!
How much bog are you using ??
how are you measuring your catalyst ??
its polyester!!
so that easy !!

Carbosil is a thickening agent !! Mixed with resin just thickens thats all it does !!!! It has no guts what so ever and is totally useless apart from what i just said its there for!!

Microballoons or Q cells again is totally useless !! again has no guts what so ever !! Is hollow spheres and full of nothingness . makes light weight easy sand filler that should be in a thin layers !!

If you have a lot of bogging to do mix a container enough to do all the work . just take out whats required and then catalyse just tipping the catalyst in is not measureing accurately enought
You have to use some scales and know the weight and what percentage of catalyst you going to use .

In combination carbosil /microbaloons is only good for thin fareing maybe depending on what kind of a recipy you using .
If you are having problems with fast and slow your ratio of catalyst is all up the shoot . how are you weighing anything or just bit of this bit of that and a splash of that and she'll be right MAYBE ??
Do you mix a quanty and then take some off and mix and use or you just mixing and catalysing as you go ??

Cracking is because of everything you doing is guess work !! and the materials are wrong as well . just those two together are as usless and **** on a bull for coving !!
If you got leaks its no wonder it needs to be completely dug out and glassed first , then coved and then totally glassed over again .

Remember when you have the resin in a conainer and then mix carbo sill and then microballoons you are slowing the reaction time of the original resin everytime you add more mix !!!the more you mix in the slower it becomes !!,so when its really stiff it probably wont go hard at all because you have killed it !!
Anothr bad thing with Microbaloons is it separates out of the resin and if left standing in a container the resin will gadually completely separte out so there just sticky microbaloons with next to no resin at all .

Can you see where you going wrong with everything you doing ??

Ok a mix for coving should be resin , Talc , carbosil and micro fibres .and deffinitly no qcells for anything so dont even think about it .
I mix about a litre of resin in a container and use a measure like a small baked been tin one of talc , one of carbosill and 1/3 microfibers then mix it . should still be sloppy so add a small amount of tal and carbosill to stiffen the brew to its ok to cove with , when the bog touchs a surface it should leave a resin foot print if it dosent then it will never stic to anything at all .
USE SOME SCALES AND PLACE THE BOARD ON TOP THEN ADD HOW EVER MUCH OF THE MIX YOU WANT TO USE !! The catalyst ratio needs to be colse to 1.5% of the weight of bog you have !! make a crater in the middle and pour in the catalyst and mix completely !!
Temparture and humidity play a big part in the gel times of all and any resins so if a surface is wet or even damp its never going to go hard . if its cold warm it up a little if is wet dry it .


Its a everchaging world and what worked yesterday may not work today because of incompatability of products and changes in products we never get told about till we have problems !!!

 

Thanks for your advice.
The radius I'm using to bond bulkheads is about 10mm, so not a lot of volume in Tasmania's cool climate.
I use a 10 litre container to mix a large volume of uncatalysed bog, the ratio being about 50/50 resin / cabosil by volume, depending on the job I'm doing.
I add catalyst to small volumes using a measuring bottle, which gives very accurate and consistent results. Well, it has until now and it's only the polyester / cabosil combination that is inconsistent.
I could talk to the people who sell the stuff or the manufacturers but I'd expect a snow job.
Thanks, Harry

PS My brain was in neutral. The mix was 50/50 resin / cabosil, not 50/50 resin / catalyst.

 

50/50 is epoxy !! yes ?? and always remember there is never any about with catalyst or hardener mixing !!! its always procise or not at all !!

10MM RADIUS for bulkheads is way to small !!, so thats a 20mm dia pipe size ?? 30 mm is much better and easyer to roll the glass round and far stronger and makes a much better job and if the bog is pressed well in makes a really good neat cove !!

 

well did you get the answers you were seeking ??

 

little note.....for those doing corners, when using , say 30mm pipe, mask 15mm out from the walls, (the contact point), do your little thing screening it, remove the tape, and the end result is a perfect radius with no clean up at all.

 

Hi Tunnels
Thanks for your follow-up.
I'm just as wise about the cracking bog. That means I haven't got a clue. That's why I asked.
I'm quite experienced as a builder, forty years or so but this is a new problem for me.
Hopefully, someone out there will have experienced the same thing.
Until then, the boat is a success, quite fast, no bad habits and fun to sail.
Harry